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They Discovered an 11,000-year-old Submerged Ochre Mine

The exploration crew at CINDAQ, headquartered at Zero Gravity Dive Center in Puerto Aventuras made international news this year with their discovery of an ancient submerged ochre mine. Fortunately, they were happy to share the secrets of its discovery and how they documented their find with British cave and 3D photogrammetry instructor John Kendall. Oculus Rifts anyone?

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By John Kendall
Header image courtesy of
CINDAQ

In 2017, underwater cave explorers Fred Devos, Christophe Le Maillot, and Sam Meacham found evidence of ancient mining activity while exploring and mapping new tunnels of an underwater cave near Akumal, Quintana Roo, Mexico.​ Historians know that ancient residents actively mined pigment and other minerals from the caves of the Yucatan Peninsula, but the ancient mines the CINDAQ team discovered are now submerged, indicating that such mineral exploitation occurred thousands of years ago. 

At the end of the last Ice Age, intrepid miners ventured deep into these tunnels with torches in hand. The navigational markers, mining debris, fire pits, and excavation pits they left behind are now entirely underwater. Over the last three years, the three explorers (along with others) have been surveying the site and making 3D photogrammetric models of the mine workings. As the mine has been submerged for around 8,000 years, it’s been untouched since then, and it’s an amazing time capsule. The project recently hit the international news when the first results were published. I was pleased to be able to chat with Chris, Fred, and Sam to find out a bit more about the project, and the challenges faced with archaeological work in a cave environment.

The InDepth Word Cloud: A map of the frequency of words spoken in this interview.

John Kendall: How did you happen to find the mine?

Fred Devos prepping for a dive.

Chris Le Maillot: As always, there was a little bit of chance involved with it. The cave—Sagitario, which is a beautiful cave behind Minotauro—was initially explored by a few local cave divers. They established an upstream and part of a downstream, dropping down in the upstream to around 22 m/72 ft, and there’s the halocline sitting at that depth. It’s not always the case, but they didn’t take any survey, absolutely nothing. So I don’t think the information was there for them to continue on with the exploration. As you know, once you have that data in and have a good concept of what the cave is doing and where it’s going, it’s easier for you to poke around and find potential continuations of the cave passages. 

Chris Le Maillot.

So one of the divers asked Fred [Devos] to get involved to create a survey. That comes from the fact that Fred previously had done some mapping for these guys. Fred had a cave survey class coming up, so he took the class there, and spent the week with the survey class mapping the downstream part. Obviously, when they got to the end of the line, Fred could see that there was potential for further exploration. But you can’t really go off exploring during a class, so he went back with Sam [Meacham].

Sam Meacham before a dive with the team in La Mina.

So then you went back and explored?

Fred Devos: Exploring caves is what we’ve been doing for more than 20 years, and so it’s a regular event during the mapping of a cave to find more cave to explore. You know, when mapping, we have to swim off to measure the side walls, sometimes there isn’t a wall, and then end up exploring that passage. I was in the process of making a detailed map of this cave, and found this passage, so I went back with Sam, and we immediately realized something was unusual. Things were out of place, we started seeing rocks piled on top of each other, speleothems in places they shouldn’t have been, and the further we went the more of this we saw. 

“Exploring caves is what we’ve been doing for more than 20 years, and so it’s a regular event during the mapping of a cave to find more cave to explore.”

We started picking up a little bit of flow, which is always a good thing in exploration, and that led us to this restriction, where all the water was going through, and I don’t think we’d have made it through if the restriction hadn’t been manipulated before we got there. So, you know, speleothems were smashed out, and it really looked like 100 divers had gone through there before us, which really piqued our curiosity as we knew no one had been there before us. We happened to be in back mount during this dive and I managed to squeeze through there and called Sam through, and that was when we first saw irrefutable evidence of what humans had been doing in this cave—you know, pre-8,000 years ago. 

“It was pretty clear to anyone what we were seeing, that people had been digging in here, smashing open the floor and pulling out huge amounts of sediment and piling stuff out of the way. It was super exciting.”

An ochre extraction pit found by divers from Centro Investigador del Sistema Acuífero de Q Roo (CINDAQ A.C.) in the oldest ochre mine ever found in the Americas, dating back 10,000-12,000 years. Photo courtesy of CINDAQ.

We didn’t have to wait for lab results to come back or ask an archaeologist about it. It was pretty clear to anyone what we were seeing, that people had been digging in here, smashing open the floor and pulling out huge amounts of sediment and piling stuff out of the way. It was super exciting, as it was something we’d suspected for quite a while but had never really determined for sure that was what we were seeing. But this time it was obvious, and there was no question about it. 

So, how large an area does the mine occupy?

Sam Meacham: It’s about 250 m/817 ft of cave passageways that are exemplary of the mining activity, and everything we’re seeing there shows the things that people were doing in the mine. 

Devos: And we haven’t finished exploring yet. There are hectares of mining area, so it’s not just one hole that’s been dug out. It’s entire passages and we’re talking about hundreds, maybe thousands of tons of material, and remember we have dates spanning maybe a 2,000-year period.

What makes La Mina so significant from a scientific point of view?

Devos: The amount of workings means that this was a massive undertaking. Not just the mining itself, but it’s clear it wasn’t just a one-person adventure. It must have been multi-generational, but beyond that it speaks very much about the organization of the people of that time. So as you can imagine, they were in a dark cave and needed fire for light. So they needed people to bring in the firewood, and others to cart out the material, and there were probably explorers at the time. You know, people that ventured further into the caves away from the exit into the smaller passages…to find this very valuable resource. And I imagine they were the ones that were being punished somehow because the risk involved was probably much greater. So, you know, if they didn’t do their work well in the mine, they probably got sent to explore.

A diver making notes as he examines an ochre extraction pit . Photo courtesy of CINDAQ

So are there any archaeological signs on the surface around the mine?

Devos: Well there probably are, there’s certainly Maya era archaeology, and in almost every cave we see evidence of that, but we’re talking about 5,000 years ago. The mine was even further back, so anything that was once there won’t be anymore, and the only place we are likely to find anything is in the caves.

Let’s chat about the photogrammetry side. More and more people are hearing about photogrammetry, but I think the readers will be interested to hear a bit more about the challenges that you faced doing photogrammetry in a cave environment, where everything around you is archaeological.

Meacham: I think that can get us started on an interesting concept. In 2010, Chris, Fred, and I, Beto Nava, as well and Franco Attolini and Danny Riordan and Roberto Chavez, all did our underwater archeology course here in Mexico with the Nautical Archeology Society that was supported by the National Institute of Anthropology and History (INAH). It empowered us. 

And by having that NAS certification, it kind of helped check a box for the Institute. And, you know, they could say if anybody questioned our abilities, well, we’ve got the certification. 

I’d say the genesis of this for all of us here was the Hoyo Negro project, and with the exploration followed by the high grade survey, and then the photogrammetry, which is another whole level in itself. The major problem in Negro is the pit itself—it’s just immense—and how do you document something like that? So we worked with Beto and the team who came up with a grid system at 34 m/110 ft depth, and then it’s every 0.8 m/2.5 ft with a cookie on the line, and so it’s a systematic grid. The difficulty there is that it’s not just a nice flat bottom, it goes from 40 m/130 ft to about 55 m/179 ft, and it just becomes really complex. 

But basically what I’ve been doing there is assisting with the lighting or helping Beto. So when we jump forward to doing the mine, it’s a completely different environment. There’s no pit—it’s a continuous cave—so there was no way we could put in a grid, and I’ve never really done photogrammetry before. I had observed it being done, but I was starting from scratch in terms of my own experience. So it was a challenge, but I had plenty of people to go to as resources, and who could check out what I’d done and help make it better. And what’s interesting about the big model is that you can see my progression as we go around, and now of course I want to go back and do it all over again.  

So in terms of the challenges, I bought a Sony A7S camera and a Nauticam housing for it, and we just went in and started taking a bunch of photographs, came back, and put it into Agisoft. I have to say my expectations were low, but we were all pleasantly surprised when the model came back. This is like, “Wow that’s what we’re actually seeing there,” and it’s so cool. So that gave me the confidence to say, “I think I can do this,” and we basically picked about 250 m/817 ft of cave passage, which is a great example of the mining activity and of seeing what people were doing there. 

That sounds like quite a learning curve, and a big challenge. 

Meacham: Yes, we just started going in and piece by piece doing sections of the cave. I can’t remember how long in total we were down there. I’m sure it’s written down somewhere, but we took something around 18,000 photos. And as you know, taking the photos is probably the easiest part. Having the computing power and post-processing of the images is the key. A lot of people treat Agisoft as a bit of a black box, but you know it’s garbage in, garbage out. So in terms of the environment, we’re talking about a ceiling height that’s minimal, and while you can fit through OK, you want to be as high as possible for the photogrammetry in order to cover more area. 

“I’m sure it’s written down somewhere, but we took something around 18,000 photos. And as you know, taking the photos is probably the easiest part. Having the computing power and post-processing of the images is the key.”

So, we just worked section by section, using the line as a reference. I was going down the line and started by making sure that I got any markers on it, and then going back and forth to get all the photos. The person that suffered the most was whoever was assigned to dive with me, as they just had to sit there and watch me go back and forth while taking the photos.

18,000 images! That’s a whole lot of processing.

Sam Meacham: Yes, we’re lucky to have the guys at University of California at San Diego (UCSD) helping us with the processing. I probably started off taking too many photos, but the computer guys complimented us on the photos and the overlap and coverage. 

A landmark of piled stone and broken speleothems left 10,000-12,000 years ago by the earliest inhabitants of the Western hemisphere to find their way in and out of the mine. Photo courtesy of CINDAQ.

So what about other survey techniques, was there anything special about mapping this site?

Devos: We surveyed the first part of the cave, and that was pretty normal, but once we found the mine, then suddenly we had a need for all these new types of symbols that didn’t exist before for cave survey. I tried to think about what would be interesting to make notes of, but I didn’t want to speculate as to whether something was a natural pit or whether it was digging. 

The mine holds some of the best-preserved evidence of the earliest inhabitants of the hemisphere. Photo courtesy of CINDAQ

So we came up with three new symbols. There was already a symbol for a pit, but we added a jagged line on the pit to show that there was a broken edge, so it was smashed. Then we came up with a symbol for a displaced object, so if you see some stalactites and there was no way it came from the ceiling above, then that’s a displaced object. And then if you have stacked objects, so objects placed on top of each other, we had a symbol for that. We then made all of these colored red. I chose red because of the extracted material, the ochre. Also, when you look at the map, and you see all that red, it really shows the extent of the manipulation of the cave. It really brings it out, and I think that’s the most important thing about this cave. Sidewall information is nice, but this is very much an archaeological site.

So what’s next with the site? Any further diving plans?

Devos: We have some plans in place. The map that we’ve made, the photogrammetry, and the video documentation, even the exploration are not finished. So we actually concentrated on one area and tried to get that in the bag, you know, and focus our studies and our samples in that area, without stretching too far, but there’s still a huge portion to go. The technology really helps here, because you can bring that information out for the scientists and others to see. And then there’s much less need for others to go back there. 

And this is really the part where we don’t know what’s going to happen. Are divers one day going to be able to go there to tour this site? Luckily, I’m not the one who will be making that decision; there is an archaeological department in Mexico who set the rules. But these conversations are starting, and we’re not really sure where they will lead. But for now we are doing what we can to secure the documentation of the site and working closely with the archaeologists and the landowner. 

A CINDAQ diver noting details of the mine. Photo courtesy of CINDAQ.

So a last question: What would your advice be to a diver who is just starting out on their GUE journey, and who hears about this and other projects, and wants to one day join?

Devos: We have been running all kinds of projects down here for years: exploration, science, surveys. Come and get involved, and help out. Good basic training helps open up the door. 

Meacham: Once you’ve trained and gained enough experience to become confident in whatever environment you’re interested in, then come and get involved. There’s great training with the GUE Documentation Diver program, Science Diver, Photogrammetry Diver, and Cave Survey where you can actually put these skills to the test. Everyone on a project is an important part of making it work. Obviously it becomes tricky when archaeology is involved, as there can be federal laws and regulations that restrict access, and so we can’t always put just anyone onto a site, but there are all sorts of projects within GUE to help develop those skills and get known by project leaders.

Le Maillot: Of course, project diving is what GUE has been known for since the very beginning. So I think making that initial step to take training with GUE is an important one in the right direction. That’s the starting point of understanding how we are organized, the procedures that we use, [and] the team aspects of all our diving. And then it’s about thinking about what you want to do. 

“Of course, project diving is what GUE has been known for since the very beginning. So I think making that initial step to take training with GUE is an important one in the right direction.”

If you’re interested in wrecks, you have Mario Arena in Sicily or Richard Lundgren with the Mars project, and you’re naturally going to be headed down the Tech 1/CCR route. If it’s the stuff in Florida, or Bosnia, or here in Mexico, and the cave thing really rocks your boat, then that’s where the GUE cave training comes in. Then, as you progress with your tech or cave training, you will get to know divers who are involved in projects, and that could be your instructor. You know, if you come here to do some cave diving in Mexico, then Fred is going to mention a few things about survey and cave projects in Mexico and around the world. So that will start opening up a different perspective for you. 

Additional Resources:

InDepth: Data for Divers: Mexican Explorers Go Digital to Chart Riviera Maya

Watch a Video of the Mine on GUE.tv. (Requires a GUE.tv membership or signing up for a free trial)

For more information about the La Mina project, you can visit the CINDAQ website

Check out the CINDAQ YouTube channel


John Kendall is a GUE technical, cave, and CCR instructor living in the UK. Since he was a small child, John has been fascinated by the underwater environment and the possibilities of adventure, and he is grateful to GUE for helping him to turn those childhood dreams into reality. As an instructor, John regularly travels around the world teaching GUE classes and helping to build local GUE communities. For the last 5 years, John has been working with underwater 3D Photogrammetry as a technique for nautical archaeology. This cutting edge technique allows for digital 3D models to be created of shipwrecks and caves, and allows researchers and scientists unparalleled abilities to manipulate and navigate the sites from the comfort of their own computers. John was the primary author of the GUE Photogrammetry class. He is also a member of the GUE Training Council and a Fellow of the Explorers Club.

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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