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Capturing 3D Audio Underwater

Listen up divers! Audio engineer turned techie, Symeon Manias is bringing 3D audio to the underwater world to provide a more immersive and convincing auditory experience to listeners whether its video, film, or virtual reality. So, pump up the volume—here’s what you need to know.

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by Symeon Manias
Header image by Peter Gaertner/Symeon Manias and edited by Amanda White

Spatial hearing provides sound related information about the world. Where does a sound come from? How far away is it? In what space does the sound reside? Humans are pretty good at localizing sound sources and making sense of the world of sounds. The human auditory system performs localization tasks either by solely relying on auditory cues or by fusing them with visual, dynamic, or experience-based cues. Auditory cues consist of cues obtained by one ear, which is termed monaural, or by using both ears, termed binaural. 

Humans can detect sounds of frequency content between 20 Hz and 20 kHz. That translates to wavelengths between 1.7 cm and 17 m. This frequency range decreases with age as the internal parts of the ear degrade; however, exposure to loud noises can accelerate this degradation. The peripheral part of each of the human ears, which by no coincidence is common to most mammals, consists of three structures. The external part of the ear, the middle ear, and the inner ear. Figure 1 shows in detail all these parts of the ear along with their basic structure

Figure 1.
Courtesy of Evangelos Pantazis.

The external ear consists mainly of the pinna, the concha, and the ear canal and ends at the eardrum. The pinna and concha, which are visible as the outer part of the ear, are responsible for modifying the frequency spectrum of sounds coming from different directions. The ear canal, on the other hand, is responsible for enhancing specific frequencies of most incoming sounds. The ear canal can be seen as a resonant tube with a resonant frequency that depends on the length and diameter of the tube. For most humans, the resonant frequency of the ear canal is between 2 kHz and 4 kHz. This means that all sounds that travel through the ear canal will be enhanced in that frequency range. Airborne sounds travel from the world through the pinna, the concha, the ear canal, and hit the eardrum, which is at the end of the ear canal. The eardrum separates the outer from the middle ear and acts as a transformer that converts sounds to mechanical vibration. 

On the other side of the eardrum, in the middle ear, there are three ossicles, the malleus, the incus and the stapes. These bones are responsible for transmitting the sounds from the outer ear through vibration to the entrance of the inner ear at the oval window. This is where mechanical vibration is transformed into sound to then be received by the sound receptors in the middle ear. 

One might wonder why we even need the middle ear if the incoming sound is first transformed at the eardrum into mechanical vibration and then back to sound at the oval window. If the middle ear was absent, it is very likely that incoming sound would be reflected back at the eardrum. Another reason is that the middle ear generally acts as a dynamics processor; it has a protection functionality. When a loud sound travels through the ear canal and hits the eardrum, the muscles that are attached to the middle ear ossicles can attenuate the mechanical vibration. This is a phenomenon known in psychoacoustics as an acoustics reflex. The middle ear also serves as a means to reduce the transmission of internally generated sounds that are transmitted through bone conduction, as well as letting air be transmitted on the back side of the eardrum through the Eustachian tube, which balances the pressure that is building on the eardrum.

The inner ear is a very complex structure which is responsible for two functions: one is the vestibular system, which is responsible for our orientation and balance in the three-dimensional world, and the other is the cochlea, which is dedicated to sound perception. The cochlea, which looks like a hard shell, is essentially a hard bone filled with fluid and consisting of rigid walls which continue like a spiral with decreasing diameter. Sounds enter the cochlea through the oval window and cause the liquid inside the cochlea to begin to ripple, and that’s where the magic happens. 

There are two membranes inside the cochlea, the vestibular membrane, which separates the cochlear duct from the vestibular duct, and the basilar membrane. The basilar membrane is where the organ of Corti resides; this area consists of two sets of sensory receptors, named hair cells, and these are the inner and outer cells. The inner hair cells, which move as the incoming sound waves travel through the cochlear fluid, act as transducers that transform the motion into neural spike activity that is then sent to the auditory nerve. 

The outer hair cells can influence the overall sensitivity of each perceived sound. The neural spikes that travel through the auditory nerve are then received by the brain and are interpreted as sounds that we know and can understand. Different sections of the basilar membrane inside the cochlea are responsible for interpreting different frequencies of sound. The section near the entrance consists of hair cells that are responsible for the interpretation of high pitched sounds and, as we move further inside the cochlea, detect progressively lower pitched sounds.

Sound Source Localization

A single ear is basically a frequency analyzer that can provide information to the brain about the pitch of a sound with some dynamics processing that can protect us from loud sounds and some basic filtering of a sound that comes from different directions. So what can we do with two ears that it is almost impossible to do with one? Two ears can provide information about another dimension—the direction—and consequently can give us an estimate of the location of a sound. This is termed spatial hearing. The main theories about how spatial hearing works developed around the idea of how we localize sounds with two ears and get information about the environment in which they reside.

The human auditory system deploys an assortment of localization cues to determine the location of a sound event. The underlying human localization theory is that the main auditory cues that assist with localization are the time and level difference between the two ears and the shape of the ear itself. The most prominent auditory cues use both ears for determining the direction of a sound source and are called binaural cues. These cues measure the time difference of a sound arriving in the two ears, named as interaural time difference (ITD), and cues that track the level difference between the two ears, named as interaural intensity difference (IID). Sound arrives at different times between the two ears and the time delay can be up to approximately 0.7 milliseconds (ms). ITD is meaningful for frequencies up to approximately 1.5 kHz, while ILD provides meaning for frequencies above 1.5 kHz. See Figure 2 for a visual explanation of the ITD and ILD.

Figure 2.

ITD and IID alone cannot adequately describe the sound localization process. Imagine a sound coming from directly in front vs the same sound coming directly from the back. ITD and IID provide the exact same values for both ears since their distance from the sound is exactly the same. This is true for all sounds that are on the median plane, which is the plane defined from the points directly in front of us, then up and then back. This is where monaural cues play a role. These are generated by the pinna, diffraction, and reflections of our torso and head. The frequencies of sounds coming from our back or on top of our head will be attenuated differently than sounds coming from front. Another set of cues are dynamic cues which are generated by moving our head. These can show the effect of our head movement to ITD and IID. Last but not least, visual and experience-based cues play an important role as well. Our visual cues, when available,  fuse information with auditory cues and resolve to more accurate source localization.

Localizing Sound Underwater

If, under certain conditions, humans can localize a sound source with accuracy as high as 1 degree in the horizontal plane, what makes it so difficult to understand where sounds are coming from when we are underwater? The story of sound perception becomes very different underwater. When we are submerged the localization abilities of the auditory systems degrade significantly. The factor that has the biggest impact when we enter the water is the speed of sound. The speed of sound in air is approximately 343 m/s, while underwater it is 1480 m/s and depends mainly on temperature, salinity etc. A speed of sound that is more than 4x higher than the speed of sound in air will result in sound arriving in our ear with much smaller time differences, which could possibly diminish or even eliminate completely our spatial hearing abilities. 

The perception of time differences is not the only cue that is affected; the perception of interaural intensity differences is also affected by the fact that there is very little mechanical impedance mismatch between water and our head. In air, our head acts as an acoustic barrier that attenuates high frequencies due to shadowing effect. Underwater sound most likely travels directly through our head since the impedances of water and our head are very similar. and the head is acoustically transparent. These factors might suggest that underwater, humans could be considered as a one-eared mammal. 

In principle, the auditory system performs very poorly underwater. On the other hand, there have been studies and early reports by divers that suggest that some primitive type of localization can be achieved underwater. In a very limited study that aimed to determine the minimum audible angle, divers were instructed to perform a left-right discrimination and were able to improve their ability when they were given feedback on their performance. 

Another study was conducted where various underwater projectors were placed at 0, +/-45 and +/-90 degrees at ear level in the horizontal plane with 0 degrees being in front of the diver in [5]. The stimuli consisted of pure tones and noise. The results indicated that divers could perform enough localization that it could not be written off as coincidence or chance. It is not entirely clear whether the underwater localization ability is something that humans can learn and adapt or if it’s a limitation of the human auditory system itself.

Assistive Technologies for Sound Recording

In a perfect world, humans could perform localization tasks underwater as accurately as they could in air. Until that day, should it ever come, we can use assistive technologies to sense the underwater sound environment. Assistive technologies can be exploited for two main applications of 3D audio underwater: localization of sound and sound recording. Sound sensing with microphones is a well developed topic, especially nowadays since the cost of building devices with multiple microphones has decreased dramatically. Multi-microphone devices can be found everywhere in our daily life from portable devices to cars. These devices use multiple microphones to make sense of the world and deliver to the user improved sensing abilities but also the ability to capture the sounds with great accuracy. 

The aim of 3D audio is to provide an immersive, appealing, and convincing auditory experience to a listener. This can be achieved by delivering the appropriate spatial cues using a reproduction system that consists of either headphones or loudspeakers. There are quite a few techniques regarding 3D audio localization and 3D audio recording that one can exploit for an immersive experience. Traditionally, recording relies on a small number of sensors that are either played back directly to the loudspeaker or are mixed linearly and then fed to loudspeakers. The main idea here is that if we can approximate the binaural cues, we can trick the human auditory system into thinking that sounds come from specific directions and give the impression of being there. 

One of the earliest approaches in the microphone domain was a stereophonic technique invented by Blumlein in Bell Labs [6]. Although this technique was invented in the beginning of the last century, it took many decades to actually commercialize it. From there on, many different stereophonic techniques were utilized that aimed at approximating the desired time and level differences to be delivered to the listener. This was performed by changing the distance between the sensors or by placing directional sensors really close together, and delivering the directional cues by exploiting the directivity of the sensors. The transition from stereophonic to surround systems came right after by using multiple sensors for capturing sounds and multiple loudspeakers for playback. These are, for example, the traditional microphone arrangements we can see on top of an orchestra playing in a hall.

A more advanced technology that existed since the 1970s is called Ambisonics. It consists of a unified generalized approach to the recording, analysis, and reproduction of any recorded sound field. It utilized a standard microphone recording setup with four sensors placed almost in a coincident manner on the tetrahedral arrangement. This method provided great flexibility in terms of playback: once the audio signals are recorded they can be played back in any type of output setup, such as a stereophonic pair of loudspeakers, a surround setup, or plain headphones. The mathematical background of this technique is based on solving the acoustic wave equation, and for a non-expert this can become an inconvenience to use in practice. 

Fortunately, there are tools that make this process straightforward for the non-expert. In practical terms, this technology is based on recording signals from various sensors and then combining them to obtain a new set of audio signals, called ambisonic signals. The requirement for acquiring optimal ambisonic signals is to record the signals with sensors placed on the surface of a sphere for practical and theoretical convenience. The process is based on sampling the sound field pressure over a surface or volume around the origin. The accuracy of the sampling depends on the array geometry, the number of sensors, and the distance between the sensors. The encoded ambisonic signals can then accurately describe the sound field. 

By using ambisonic signals, one can approximate the pressure and particle velocity of the sound field at the point where the sensors are located. By estimating these two quantities we can then perform an energetic analysis of the sound field and estimate the direction of a sound by using the active intensity vector. The active intensity vector points to the direction of the energy flow in a sound field. The opposite of that direction will point exactly where the energy is coming from, therefore it will show the direction of arrival. A generic view of such a system is shown in Figure 3. For more technical details on how an three-dimensional sound processor works the reader is referred to [3,7].

Figure 3

For underwater audio-related applications, a similar sensor is used, namely the hydrophone. The signal processing principles that can be used underwater are very similar to the ones used with microphones. However, the applications are not as widely spread. Multiple hydrophones, namely hydrophone arrays, have been utilized for localization tasks, but many of them are based on classical theories that require the hydrophones to be very far apart from each other. Therefore, compact, wearable devices are uncommon. A compact hydrophone array that can be carried by a single diver can serve multiple purposes: it can operate as a localization device that can locate quite accurately where sounds are originating from, and it can also capture the sounds with spatial accuracy that can later be played back and give the experience of being there. This can be especially useful for people that do not have the chance to experience the underwater world at all or listen to the sounds of highly inaccessible places where diving is only for highly-skilled technical divers.

A purposefully built compact hydrophone array which can be operated by a diver and that can perform simultaneously a localization task but also record spatial audio that can be later played through a pair of headphones, is shown in a recently published article as a proof of concept. It is suggested that a simple set of four hydrophone sensors can be exploited in real-time to identify the direction of underwater sound objects with relatively high accuracy, both visually and aurally. 

Figure 4.

Underwater sound localization is not a new technology, but the ability to do such tasks with compact hydrophone arrays where the hydrophones are closely spaced together has the potential for many applications. The hydrophone array used in the study discussed above, was an open-body hydrophone array, consisting of four sensors placed at the vertices of a tetrahedral frame. A photo of the hydrophone array is shown in Figure 4. More technical details on the hydrophone array can be found in the following publication [2]. The hydrophone array was used to provide a visualization of the sound field. The sound-field visualization is basically a sound activity map where the relative energy for many directions is depicted using a color gradient like in Figure 5. These experiments were performed in a diving pool. 

The depth of the hydrophone array was fixed by attaching it to the end of a metal rod, which was made out of a three-piece aluminum tube. The hydrophone array signals were recorded and processed in real time with a set of virtual instrument plugins. The observer, a single action camera in this case,  was able to clearly track the movements of the diver from the perspective of the array, both visually and aurally. The diver from the perspective of the array and the corresponding activity map are both shown in Figure 5. A video demonstrating the tracking and headphone rendering with a diver inside the swimming pool can be found here.

Figure 5.

This example has been utilized as a proof of concept that compact arrays can be indeed exploited by divers simultaneously for sound visualization and accurate three-dimensional sound recording. In the examples shown, the sound arrives at the hydrophone array and then the user can visually see the location of sound and simultaneously listen to the acoustic field. The three-dimensional audio cues are delivered accurately and the sound sources are represented to the user in their correct spatial location so the user can accurately localize them. 

3D Video Recording

The current state of the art in 3D video recording underwater usually captures the audio with the microphones built into the cameras. Although sounds are often recorded in this way, the recorded sound through built-in microphones inside a casing cannot capture underwater sounds accurately. The development of a relatively compact hydrophone array such as the one demonstrated could potentially enhance the audio-visual experience by providing accurate audio rendering of the recorded sound scene. Visuals and specific 3D video recordings can be pretty impressive by themselves, but the addition of an accompanying 3D audio component can potentially enhance the experience. 

Visuals and specific 3D video recordings can be pretty impressive by themselves, but the addition of an accompanying 3D audio component can potentially enhance the experience.

Ambisonic technologies are a great candidate for such an application since the underwater sound field can be easily recorded with an ambisonic hydrophone array (such as the one shown in Figure 4) and can be mounted near or within the 3D camera system. Ambisonics aim at a complete reconstruction of the physical sound scene. Once the sound scene is recorded and transformed in the ambisonic format, a series of additional spatial transformations/manipulation can come handy. Ambisonics allow straightforward manipulation of the recorded sound scene, such as a rotation of the whole sound field to arbitrary directions, mirroring of the sound field to any axis, direction-dependent manipulation such as focusing on different directions or areas, or even warping of the whole sound field. The quality of the reproduced sound scene and accuracy of all the sound scene manipulation techniques highly depends on the number of sensors involved in the recording.

Some modern, state-of-the art techniques are based on the same recording principle, but they can provide an enhanced audio experience by using a low number of sensors that exploit human hearing abilities as well as advanced statistical signal processing techniques. These techniques aim to imitate the way human hearing works, and by processing the sounds in time, frequency, and space, they deliver the necessary spatial cues for an engaging aural experience, which is usually perceived as very close to reality. They exploit the limitations of the human auditory system and analyze only the properties of the sound scene that are meaningful to a human listener. Once the sound scene is recorded and analyzed into the frequency content of different sounds, it can then be manipulated and synthesized for playback [7].

Additional Resources


Dr. Symeon Manias has worked in the field of spatial audio since 2008. He has received awards from the Audio Engineering Society, the Nokia Foundation, the Foundation of Aalto Science and Technology, and the International College of the European University of Brittany. He is an editor and author of the first parametric spatial audio book from Wiley and IEEE Press. Manias can be usually found diving through the kelp forests of Southern California.
Contact: symewn@gmail.com

Cave

Meet The British Underground

It’s cold, dark, you can barely see two meters in front of you, and you’re diving alone. Oh, and there’s a sump up ahead. Welcome to the British Underground! Not exactly a scooter-ride in the warm, clear, stalactite studded caves that lay beneath Riviera Maya. Here British caver and training officer for the Somerset Section of the Cave Diving Group Michael Thomas guides us on a tour of British cave diving and explains why it may not be everyone’s cuppa tea.

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by Michael Thomas 

Header photo courtesy Michael Thomas. Diver entering Keld Head in the Yorkshire Dales.

Recently someone approached me about British cave diving wondering what in particular makes it so very different from Mexican cave diving, for example, and why it’s so appealing to a select few. In the U.K. we have two types of underground diving. The first is the significant number of flooded mines that have given rise to some world-class mine diving that’s becoming very popular with technical divers from around the world. The second type of  underground diving is traditional British cave diving, which, due to the nature of U.K. caves, involves both dry caving and cave diving. The aim is to explore the caves underwater or in the dry underground following it as far as possible. We are now finding that technically trained mine and cave divers are starting to learn the art of dry cave exploration in order to further their knowledge and adventure, some even gaining enough experience to join the Cave Diving Group in the U.K. 

Firstly, a little about myself if I may be so bold. My diving career is now in its thirty-third consecutive year, from starting out as a trainee open water diver with BSAC to trainee cave diver within the CDG to becoming the Training Officer of the British Cave Diving Group Somerset Section in the U.K. Since 1996 I’ve had links to TDI and currently hold Full Cave Instructor, Sidemount and Tech Instructor status with TDI, active mod 3 CCR cave diver, and on the British Cave Rescue call out list as a diver. 

My diving life crosses all paths of British and worldwide diving, from open water to cave and tech. I’m actively involved with technical diving conferences and a fellow of the Royal Geographic Society of the U.K. My father was a cave explorer before me, and my son has also taken the same path. You could say, caves and diving are our lives. 

See The CDG 

To understand British cave diving we first need to understand the CDG.  The Cave Diving Group is the representative body for cave divers in Great Britain and Northern Ireland and is a constituent body of the British Caving Association (BCA). Its function is to educate and support cavers for recreational and exploratory operations in British sumps. The CDG also helps control access to numerous cave sites, including Wookey Hole and Gough’s Cave  in Somerset, and Keld Head And Hurtle Pot in Yorkshire, in conjunction with the BCA. The group was formed in 1946 by the late Graham Balcombe, and its continuous existence to the present day makes it the oldest amateur technical and cave diving organization in the world. Graham Balcombe arguably invented cave diving in the U.K. with his audacious dives in Swildon’s Hole cave and Wookey Hole cave in 1935. 

Now the huge difference between the Cave Diving Group and other cave diving training agencies is you can’t just sign up and pay to do a training course. From the very start in 1946, the prerequisite for joining the CDG was always and is a knowledge and experience base of dry caving skills, though in modern years we also require an open water certification. Once you have made yourself known to one of the four sections that make up the CDG—Somerset, Welsh, Northern, and Derbyshire—and proven you have dry caving skills and can get along with your new-found friends, you are voted in, hopefully to whichever section you approached. 

CDG rescue practice exercise after the Thailand rescue to update our rescue skills and standards. Photo courtesy of Michael Thomas.

As a trainee member of the cave diving group, the training is apprenticeship based and generally takes between 12-18 months. At the end of that, a written exam and an underwater test is completed, and as long as your section is in agreement, the qualified diver status is awarded. It’s a slow process but ensures adequate experience is gained in a variety of different sites and conditions, producing a cave diver that is capable of exploration cave diving, rescue work, and continued training of new members. 



The Solo Mentality 

Probably the one difference with most U.K. cave diving, that is a world away from agency standards, is the CDG approach to team diving. In all but a few sites in the U.K., the CDG considers solo diving the safest way to approach the dive. While divers might enter the cave together as a team, and dry cave their way to the dive base (dive site within the cave), once they are in the water they typically dive solo. This is because a diver is usually unable to help another diver in the water. Then they meet up on the other side if more dry caving is to be done. 

CDG trainee divers are taught from the beginning to be solo divers or work within a team as solo divers, something we call “team solo.” Most dive sites in natural caves in the U.K. are unsuitable for team diving. The few sites that are suitable for a team to operate together, such as Wookey Hole in Somerset, Hurtle Pot in the Yorkshire Dales, and Porth Yr Ogof in South Wales, should really be a team of only two divers. Passage size and visibility generally means divers can’t see the third team member if at the back or front of the team. The mine diving sites are much more suited to team diving with larger passages and clearer water. The links below offer more information on mine diving in the U.K. [Ed.note: Global Underwater Explorers does not sanction solo diving.]

Robert Thomas at Top waterfall sump dive base Ogof Ffynnon Ddu Cave Wales UK. Photo courtesy of Michael Thomas.

The article, “Solo Cave Diving,” on the CDG website explains why it recommends solo cave diving as the safer alternative for U.K. sump conditions. It lists some of the advantages of solo cave diving as follows:

  • There’s no one to get physically jammed in the passage behind you (thereby blocking your exit).
  • There’s no one behind you who may get tangled in the line and have to cut it—leaving you with no guide home.
  • There’s no one to accidentally disturb your ‘out tags’ at line junctions (e.g. in one cave there are 10 branch lines off the main line in the first 500 m/1640 ft of passage).
  • There’s no one to cause silt problems (but yourself).
  • There’s no chance of being called upon to share air—in small passages.
  • There’s nothing to get confused about—communication in sumps varies from difficult to impossible.
  • There’s no one to provide you with a false sense of security.
  • There’s no one to worry about but yourself, so you can concentrate on your own safety.

Equipment Configuration

Due to the generally small passage size of British caves and the sometimes energetic nature of transporting equipment to a cave dive base (station), sidemount diving is the normal equipment configuration. Sidemount started in the U.K. in the 1960s with a need for streamlined and lightweight diving equipment. U.K. cave divers today will have a choice of sidemount harness for the project they are involved with. In a short, shallow, or constricted dive, the diver will use a wetsuit and a lightweight, webbing-only harness with no buoyancy, as it’s not needed if your chest is on the floor and your back on the ceiling. 

 Diver leaving Dire Straits dive base Gough’s Cave Somerset UK. Photo courtesy of Michael Thomas.

In slightly larger cave passages, the modern British cave diver will use one of the now-common sidemount harnesses that are available. Many British caves require vertical cave techniques to reach the water, so the divers have modified the sidemount harness to be able to descend into the dry cave, do the dive, and then climb out. It’s very rare to find a British cave diver with an unmodified sidemount harness. For exposure suits, many short dives and some longer dives, if significant dry caving is expected before or after, the dive will be done in a wetsuit even though water temperature is on average between 7-10 degrees C/45-50 degrees F. This is for practical reasons, as it’s very difficult and potentially dangerous caving in a drysuit, so better to be slightly chilly during the dive and caving safely. 

For larger sites and when the diver is expected to be underwater most of the time, a drysuit is sometimes carried to the dive base and put on underground, to be utilized for the dive. As most divers are solo diving, having two short standard-length hoses on their regulators is normal, although for team diving or cave rescue, a standard long hose is used on the right. 

British cave divers always use helmets, as they provide protection from the environment in the dry caves as well as underwater, and are a great place to carry lights. Hand-held primary lights are used in larger, clearer passages with the helmet lights in reserve. For dive lines, we use 4 mm thick lines as permanent dive lines and we have fixed junctions in all caves— no jumps or gaps. Standard line arrows and cookies will not fit on a U.K. line. Pegs are used, or permanent markers on junctions to show the way home. It’s very unlikely you will see another dive team in the cave on the day you are visiting, and following a thicker line in sometimes low visibility and cold water is much safer and more comforting than trying to follow a technical diving line. 

Exploring U.K. caves 

The raison d’etre for the Cave Diving Groups formation was and still is the exploration of caves, including the surveying and reporting of that exploration and the training of new divers. The CDG publishes a journal four times a year with exploration reports and many books on the subject of U.K. caves and techniques. If it’s not surveyed and reported, it’s not explored. Now, it would be extremely tedious to the reader if I listed all exploration in underwater caves in the U.K.—we have thousands of reports—so I’ll mention a few of the classics to set the scene, and remember all exploration can be researched in the CDG journals. 

If visiting, most U.K. cave divers are happy to show you around or even get you involved in projects, although they will be of a very different style of exploration than found in Bahamas or Mexico, for instance, where swimming into hundreds of metres of new cave is possible. Big breakthroughs in the U.K. are rare, and if a diver explores 10 m/33 ft of new cave with ongoing passage seen, they will be happy. 

Wookey Hole Cave Somerset, Somerset 

Exploration in the home of British cave diving started in 1935 and carries on to this day. Slow and determined work by some of the great names in cave diving, including Balcombe, Martyn Farr, Rob Parker, Rick Stanton, and John Volanthen have seen this multi sump cave reach 90 m/294 ft depth beyond chamber 25 in extremely committing passages. Smaller side passages throughout the cave are still being explored. 

Michael Thomas surfacing in Wookey Hole after Recovering a 3rd 4th Century Roman pot ! Photo courtesy of Michael Thomas.

The Llangattock Cave Systems, South Wales 

Under Llangattock mountain lies many kilometers of caving—two caves, Ogof Daren Cilau at 27 km long and next door Ogof Agen Allwedd at 32.5 km, provide access to many cave diving sites that have provided incredible exploration over the years and will hopefully provide more in the years to come. One of the longest dives in the system is the Pwll y Cwm resurgence at 630 m/2066 ft long, surfacing in the downstream end of Daren Cilau. 

Kingsdale Master Cave and Keld Head, Yorkshire Dales 

One of the true classics of world class cave diving is the Kingsdale to Keld Head system. Graham Balcombe, of Wookey Hole fame, conducted dives in 1945 in Keld Head, and in 1978, Geoff Yeadon and Oliver Statham broke the world record with an 1829 m/6000 ft dive between Kingsdale Master Cave and Keld Head, connecting the two caves. In 1991, the underwater system was further extended, linking it into King Pot cave access to the valley floor, a traverse of 3 km in British conditions. Today, divers continue to explore and extend this system. 

Speedwell Cavern Main Rising, Derbyshire Dales 

This dive site requires a reasonable amount of dry caving effort to reach the dive base. The dive itself is multi-profile with a descent to 36 m/118 ft then up to 2.5 m/8 ft via a constricted rift, then finally down to 71 m/232 ft at the end. In the 1980s John Cordingley and Russel Carter worked the site and finally, 71 m/233 ft was reached by Martin Groves in 2002. The way on was lost in boulders and boiling sand with the water surging upwards. This was confirmed by John Volanthen in 2006. A change in geology and future technologies await. 

The Green Holes of Doolin, Ireland 

In the years leading up to the 1980s, open water divers reported cave entrances in the sea on the Doolin coast. These completely submerged caves are extremely weather dependent due to taking the full force of the Atlantic Ocean. But after experience gained in the Bahamas’ Blue Holes, British divers tried their luck exploring what became known as Green Holes. Several sites including Reef Caves, Hell Complex, Urchin Cave, and the longest Mermaid’s Hole have had successive and continued exploration. 1025 m/3350 ft penetration being reached in Mermaid’s Hole by Artur Kozlowski. Exploration continues when the weather allows. 

Robert Thomas demonstrates vertical caving skills needed to reach many cave diving sites in the UK. Photo courtesy of Michael Thomas.

Swildon’s Hole Cave, Somerset 

A true classic sump diver’s cave with long sections of active wet streamway takes the visitor down to a series of eight short sumps that require diving and more caving to reach the terminal sump and end of the cave at Swildon’s 12. Wetsuits and lightweight sidemount harness and small cylinders needed. A grand day out. 

Porth Yr Ogof Cave, South Wales 

One of the finest and reasonably easy physical-access cave dives in the U.K. The dive starts from a small pool after a short climb down between boulders. A shallow and comfortable passage winds its way up the valley passing Rawlbolt Airbell 150 m/492 ft from base and Four Ways airbell around 200 m/656 ft from base. At 250 m/820 ft from base, a cobble squeeze can be passed to the surface in the dry upper cave. The flow in this cave can be very high and the passage size varies from 1 m wide to 3 m wide, making progress upstream interesting and downstream on the return exciting. 

Hurtle Pot, Yorkshire Dales 

Probably the most dived cave in the U.K. due to its easy access and the possibility of longer dives upstream in a large passage towards Jingle Pot Cave and an area called The Deep reaching 35 m/114 ft depth in a low complicated passage towards the end around 460 m/1508 ft from base. Downstream a 400 m/1312 ft long traverse can be made to surface in Midge Hole cave reaching 20 m/65 ft depth on the way. This cave floods dramatically in bad weather, and constant line repairs need to be made by local CDG divers. 

Peak Cavern, Derbyshire Dales 

Peak Cavern is an extensive dry cave with several significant cave diving sites located within the system. The resurgence is a classic training dive in a lovely bedding plane style passage reaching the surface in the main cave. Ink sump within the cave, nearly 200 m/653 ft long, leads to Doom’s Retreat, an area worked by Jim Lister and the most extensive digging project to find a new cave beyond a sump in the U.K. Far Sump at 385 m/1263 ft long leads to an extensive dry section of cave with some extremely technical caving that can now get you to surface on the hills above. 

Pollatoomary Resurgence, Ireland 

Not many easy surface access cave diving sites that go deep are to be found in the British Isles, but this one in Ireland is one. A resurgence site that reaches 103 m/336 ft but in dark, unfriendly waters. Original exploration by Martyn Farr in 1978 and taken to 103 m by Artur Kozlowski. 

Divers leaving surface in Wookey Hole on a practice cave diving rescue.  photo courtesy of Michael Thomas.

In summary, British cave diving is historically one of the oldest branches of the sport of cave diving. The Cave Diving Group’s knowledge and standards and procedures evolved over the years to the safest method to explore or dive in U.K. style caves. It is not diving in Wakulla Springs and does not pretend to be, although several CDG members got involved in the early Wakulla expeditions. It is at times cold, wet, and unpleasant, but also can be extremely rewarding, with new caves found or just a superb dive in excellent conditions. U.K. style conditions can be found all over the world—think of the Thailand Rescue in 2018—and it’s in these conditions that the CDG system is at its best. If you’re wanting more information on the CDG or sump diving and vertical access sump diving, please give us a shout. Just remember—a pint of English beer is supposed to be warm. Stay safe and dive well. 

Additional Resources:

Website: The British Cave Diving Group

From GUE’s membership magazine QUEST: “British Cave Diving: Wookey Hole and The Cave Diving Group” by Duncan Price

Books about British cave diving:

A Glimmering in Darkness by Graham Balcombe 

The Darkness Beckons by Martyn Farr 

Interview with Cave Diving Legend Martyn Farr

Historical British cave diving films:

Trailer for documentary film ‘Wookey’ by Gavin Newman

The Underground Eiger (1980s)

Articles by Michael Thomas:

Slow Down Young fellas-Attitudes in Technical Diving

The Difference Between Caves and Mines

Mine Diving in the UK


Michael Thomas’s diving career is now in its 33rd consecutive year, from starting out as an open water diver then a trainee cave diver to becoming the Training Officer of the British Cave Diving Group Somerset Section. He is also a Full Cave Instructor, Sidemount and Tech Instructor with TDI, active mod 3 CCR cave diver, and on the British cave rescue call out list as a diver. 

Thomas is heavily involved in U.K. diving projects and training, plus overseas diving and caving. Diving is life or is life diving? 

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