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Hranice Abyss: The Deepest Flooded Freshwater Abyss in the World

The efforts to explore and map Hranice Abyss, located in Hranice (Přerov District) in the Czech Republic span more a century. Currently, the monstrous chasm is known to reach 384 m/1260 ft deep.

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By Michal Guba

The cave diving organization “7-02 Hranický kras,” which is a part of the Czech Speleological Society, is responsible for and has overseen the exploration of the Hranice Abyss, the deepest known flooded abyss in the world. The timeline below details the continuing exploration of the abyss. Please note that it is not easy to find divers who are technically and professionally prepared for exploration at a depth of about 200 meters/656 feet in a cave environment. 

In addition to depth, the composition of the water in Hranice Abyss can cause problems for divers. It is a mineral water (‘kyselka’ in Czech) with a high content of CO2 (carbon dioxide), which irritates the exposed parts of a diver’s body. In addition, the water’s composition has influenced the choice of diving equipment. When using open-circuit scuba, the exhalation bubbles cause a chemical reaction in the surrounding water, resulting in a rapid deterioration in visibility—it drops to zero! For that reason, open-circuit dives to depths below 50 m/164 ft were “banned” in 2001. Since then, members of 7-02 Hranice Karst have used closed-circuit rebreathers, which don’t emit bubbles, for exploration beyond 50 m/164 ft. Currently, the dive team has standardized on Divesoft’s Liberty rebreather.

1580: The first unsuccessful attempt to determine the depth of the lake at the mouth of Hranice was conducted by a breath-hold diver in 1580 and was described by Tomáš Jordán of Klauznburk. 

1900-1902: It was not until the turn of the 20th century that a professional teacher, J. V. Šindel of Hranice, repeatedly launched a weighted probe from a boat and reached a depth of 36 meters/118 feet. His findings were not challenged following exploration by geographer J. Dosedla in 1951, but they were quickly debunked by the arrival of divers and modern technologies.

1961: Bohumír Kopecký of Hranice made the very first dive in the Abyss with his handmade diving apparatus, reaching a depth of 6 meters/20 feet. 

1963: RNDr. Jiří Pogoda conducted a systematic dive survey. He found that the bottom forms a slope obliquely pointing deep under the rock massif.

1977: Miroslav Lukáš discovered the first dry space behind Zubatice. The location is called Heaven for its decoration. 

1978: Miroslav Lukáš and Jaromír Andrés discovered another dry space, named Dry Rotunda. There is a greater mouse-eared bat colony during the period from May to September.

1980:  A special glider probe by RNDr. Jiří Pogoda reached an unbelievable 260 m/853 ft.

This was followed by several dives with helium breathing mixture. 

1981: Fraňo Travěnec and Lubomír Benýšek descended for the first time with a trimix mixture to a depth of 110 meters/361 feet. After the borders opened in 1989, foreign divers also began to dive into the Abyss. 

1993: Belgian Michel Pauwels reached a depth of 155 meters/509 feet with a trimix mixture. Depth probes continue to measure areas too deep for cave divers to reach. 

1995: A remote-controlled underwater robot was first used in the Abyss. It was the ROV HYBALL, which at Lift I reached a maximum depth of 203 meters/666 feet; unfortunately, its supply cable got stuck in the fallen wood logs. The operator managed to maneuver it out but its Belgian owner, Carl von Basel, no longer wanted to continue the survey.

2000: Krzysztof Starnawski, who made the “last” deep dive with open-circuit scuba, reached a depth of 181 meters/594 feet and saw the bottom of the core of the Abyss, called Lift I. After the year 2000, divers began using closed-circuit rebreathers, which enabled them to stay longer at deeper depths. 

2003: The underwater robot ROV COLOMBO of the Main Mining Rescue Station a.s. (OKD Ostrava of the Czech Republic) was used to survey Lift I and the “New York” area. It reached a depth of 140 meters/459 feet but was limited by the length of its communication cable (150 meters/492 feet).

For a long time, it was certain that Krzysztof Starnawski of Poland and Pavel Říha saw the bottom at a depth of about 200 meters/656 feet, and that the possible continuation of the vertical direction did not lead directly to the current survey. Therefore, it was decided to provide an underwater robot rather than a diver to investigate the terrain and suggest further action. It was agreed to revive earlier collaboration with the Polish cave diver Krzysztof Starnawski.

2002-2010: Pavel Říha conducted an in-depth survey and mapped Lift I at a depth of 170 meters/558 feet.

2011:  Krzysztof Starnawski had just finished testing a unique, double closed-circuit instrument with which he dove in the Red Sea to the depth of 283 meters/928 feet. 

January 2012: Krzysztof Starnawski settled his 2000 record at 181 meters/594 feet at the Abyss. Two days later he descended to a depth of 197 meters/646 feet, creating a new depth record and discovering a narrow passage (restriction) on the north side that could perhaps be explored.

June 2012: Krzysztof Starnawski achieved an extraordinary discovery in another dive when with much difficulty, he overcame the restriction and descended to a depth of 223 meters/732 feet. By doing so, he confirmed that the Hranice Abyss continued to a greater depth than previous calculations. 

October 1, 2012: On his next dive, Starnawski descended to a depth of 223 meters/732 feet and launched a new probe, which reached 384 meters/1260 feet, setting a new Hranice depth record.

Since 2014: Members of ČSS ZO-7-02 Hranický kras have been using the Divesoft technology for exploration and research into the abyss. Specifically, the Liberty rebreather (back- and sidemount versions) enables members of ZO-7-02 Hranický kras to perform complex work activities up to 100 meters/328 feet (drilling, enlarging holes, positioning sensors, etc.), which fully utilizes the properties of the rebreather, such as low work of breathing and maintaining an optimal PO2. Members conducting dive surveys were also equipped with Freedom dive computers to ensure their safety in the complex depths and so-called “yo-yo profiles” in the Hranice Abyss, and providing for compatibility among the dive team.

June 2015: David Čani made a dive to a depth of 181 meters/594 feet (a new Czech depth record), in which he checked the status of the Lift I axis and, along with other dive participants, practiced procedures to ensure the safety of deep divers performing dives down to 200 meters/656 feet and below. 

July 2015: Krzysztof Starnawski made a dive to a depth of 220 meters/722 feet, and then launched a probe with an electronic pressure sensor. This time he measured the depth of just 365 meters/1198 feet. However, at the ascent, he made a very promising discovery when he examined a new restriction at a depth of 204 meters/669 feet and found that it opened into a passage big enough to drive a Tatra (a truck) through. This discovery was of paramount importance for the safety of divers making dives across the strait at the bottom of a massive well that was previously named as Lift I, because it might give them an alternate exit path.

August 21, 2015: Krzysztof Starnawski made a dive into another well (called Lift II), which was accessible after crossing the strait at a depth of 204 meters/669 feet and has approximately the same slope as Lift I. In this dive, Krzysztof discovered a new opening at 240 meters/787 feet (rock window) into unknown spaces, which he named “Macejko.” In doing so, Starnawski reached a maximum depth of 265 meters/869 feet, setting a new world depth cave record.

2016: The members of the Hranice Karst joined with National Geographic for the “Hranická Propast Step Beyond 400 meters” project with the help of Bartolomiej Grynda, owner of Gralmarine, to test an underwater ROV. On September 27, a new depth of 404 meters/1325 feet was reached during the Gralmarine ROV test dive, making the Hranice Abyss the deepest flooded cave in the world. The robot was again limited by the length of the communication cable of 500 meters/1640 feet. 

The ROV descended to the bottom of Lift I to the “Mikado” restriction and entered the Lift II. In Lift II, Grynda maneuvered the ROV along the cord of the measuring probe to a depth of 384 meters/1260 feet. After reaching the end, he proceeded along the wall to a depth of 404 meters/1325 feet. The robot remained “tangled” near the “Mikado” restriction in Lift I at close to 200 meters/656 feet. The robot was eventually rescued in 2017 by 7-02 Hranický kras and members of the Department of Special Diving Activities and Training from the Police Presidium of the Czech Republic.

2018: Working with the town of Hranice, the ČSS ZO 7-02 Hranický kras opened an information center at the Teplice nad Bečvou railway station detailing the current state of the Abyss exploration. There are 3D glasses available to help the tourists dive into the waters of the Abyss.

“During the dives, we discovered new irregular spaces, which will require further exploration. They are mostly deep and relatively narrow. However, none of the Hranice Karst speleologists doubt that there are still interesting discoveries to be made at Hranice Abyss,” explained 7-02 Hranický kras chairman Michel Guba.

At the moment, speleologists are working to produce maps of both the dry and flooded parts of the Abyss from top to bottom in 3 meter/10 foot increments. At the same time, photographic and video documentation is being conducted to help refine individual measurements.

To create greater awareness of the entire flooded and dry underground labyrinth to the public, all the measurements have been input into mapping programs to create a 3D model. Currently a profile of the cave is displayed on an information board showing the known spaces of the Abyss.

Currently there is an information board with a profile showing the current state of the known spaces on the observation ring near the Abyss.

Hranice Abyss Facts:

  • The deepest abyss of the Czech Republic.
  • The deepest flooded freshwater abyss of the world.
  • First written reference: 1580
  • Recorded on map: 1627
  • Cadastral area: Hranice.
  • Edge elevation of Abyss: 315 meters/1033.46 feet above sea level.
  • Entrance dry esophagus of the Abyss: length 104 meters/341 feet, width 34 meters/111 feet, depth 69.5 meters/228 feet.
  • The depth of the flooded part: 404 meters/1325.46 feet (2016).
  • The total depth of the Abyss: 473.5 meters/1553.47 feet.
  • Depth reached by divers: 265 meters/869 feet (2015).

Additional Resources:

Video of cave diver Jakub Šimánek diving in Hranice Abyss in 2017 recorded by Petr Chmel, a member of a diving club Pragoaquanaut.


Michal Guba worked as a policeman from 1992 to 2017. From 2008 to 2017, he worked as a lecturer, instructor, and deep diver. During a rescue mission (there was a huge flood in the Czech Republic in 1997), Michal decided to take a diving course at CMAS and began diving in 1998. Since 2000, he has been a part of the Hranice Abyss speleo diving team and is currently the chairman.

Michal participated as an expert (de-mine-pyrotechnic and training) on foreign missions in Switzerland, Austria, Bosnia and Herzegovina, Serbia, Croatia, and Montenegro from 2011 to 2016. Michal was awarded the Golden Rescue Cross in 2007 by the President of the Czech Republic Klaus and has received additional awards for his police work. Michael is currently employed by Czech company TRESPRESIDENTES s.r.o.

Cave

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.

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