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Laying Line in Ox Bel Ha

If you thought that the Riviera Maya had given up most of her secrets, think again. Underground expats Emőke Wagner, László Cseh, and Bjarne Knudsen managed to lay 20.3km/12.6 miles of new exploration line at Yax Chen, which is part of the Ox Bel Ha system. Here’s how they did it.

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By Emőke Wagner, László Cseh and Bjarne Knudsen

Header image: Permanent intersection at the ‘Los Pantanos’ area. Photo by Laszlo Cseh

Ox Bel Ha is the second largest underwater cave system in the world. Although it has been actively explored since 1998 by different groups of divers, the cave is steadily growing, new areas are discovered every year, and the exploration process seems unlikely to end anytime soon. Ox Bel Ha is a Mayan phrase meaning “three paths of water,” as the system expels water into the Caribbean Sea via three distinct underwater vents. The system is located southwest of Tulum in Quintana Roo, Mexico and extends southwest almost all the way to the Sian Ka’an Biosphere Reserve; a connection to the Reserve has not yet been found.

The COVID pandemic impacted Mexico, but after the local lockdown was lifted last year, our dive team decided to seek out new areas in the system and potential cave exploration. There were only the three of us: Emőke and László, (originally from Hungary but living in Mexico for some years, active GUE cave instructors) and our dear friend Bjarne from Denmark (GUE cave diver and previous WKPP member). Facing access issues immediately after quarantine, our team chose a cenote called Yax Chen (“blue or green well”) which seemed to be open to divers as if there were no pandemic at all. 

Thanks to our participation in earlier projects, we had some idea of the cave’s structure, but we’re talking about a massive section of the cave that ventures many kilometers inland, where it adds 10 km/6.2 miles of different sections to its main tunnel. It also features a lesser known and less extensive downstream section, so just getting to a new cave in an area like this can be quite a challenge.

Yax Chen was first explored in 1997 by a couple, Kay and Gary Walten, who established most of the known lines both upstream and downstream of the cenote and made incredible penetrations, especially considering the diving technology available at the time. A few years later, a group of GUE divers called the Mexican Cave Exploration Project (MCEP) continued the exploration of the area, established the connection to Cenote Far, and discovered sections like Arizona, Little Chen, and more. In 2004, Bil Phillips and Steve Bogaerts connected Yax Chen to the Ox Bel Ha cave system.

Cenote Yax Chen entrance. Photo by Emőke Wagner

Since Ox Bel Ha is a giant cave system, we decided—as always—to resurvey all existing lines, which was going to be interesting since our team had no data from the area. After a few months of survey in the early fall of 2020, we observed a lead-like opening from a more than 10-year-old line on one of our upstream survey dives; but, from our side of the tunnel, the lead looked like it hit a wall so, after swimming closer, we turned away immediately since it was too small for us to fit. 

A few days later, when we came back to almost the same spot, we were still thinking about the lead, so we decided to take another look at it. This time, instead of swimming straight to the small opening, we swam along the wall, and as we got closer, the wall turned out to be deceptive. It was more like a huge boulder that had fallen, but it had a decently sized opening on its right side. After one right turn followed by one left, the cave started to open up into a bigger collapsed room. We swam under a ledge, where the water seemed to come from, and we hit the halocline at the same time. After a 100-150 m/328-492 ft swim, the tunnel became more defined and opened up into 10-15 m/33-49 ft wide rooms with a floor to ceiling distance of 3-4 m/10-13 ft. We ran out of line quickly, knowing we had some cool exploration ahead but having no clue that this discovery would establish a new 20.3 km/12.6 mile section of the system!


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Finding New Paths

We subsequently conducted follow on dives from the newly established tunnel, located about 2 km/1 mile upstream from our entry point. We explored the cave further upstream, while documenting side tunnels and checking out other interesting leads. With the advent of electronic survey tools like Mnemo, using unknotted lines in the cave became more prevalent in local cave exploration. In the beginning of our journey, with limited access to such survey tools, we decided on using knotted lines overhead as a back up in the event of device failure. Even later in the project, when we had more access to  electronic tools, we resolved to keep putting the knotted line on our reels, since it could eventually be useful in creating a more detailed map of the cave. 

Directional markers in the newly explored area have been labeled “BEL” to represent our first initials, and “Bel” in Mayan translates to “path.” It’s very fitting, since the three of us were trying to find new paths in the cave every day.

During one of our early upstream dives in our steadily growing new area, we bumped into an existing downstream line and found a new connection between the upstream and the downstream sections of Yax Chen. After this discovery, the team decided to focus on the upstream area of the cave, continuing further northwest. 

At this point, we already had a few kilometers of cave exploration experience in other parts of the cave, but we very quickly learned that cave exploration, especially in this part of the system, was not easy. On each dive, we got progressively further from our nearest entrance. The system is very complex, it has a lot of sediment, and the tunnels don’t remain wide over long distances. We can’t count on our fingers the number of times we thought the exploration was over, decided to do one more dive, and pushed through a nasty and tight restriction to find a few more kilometers of cave.

Deep salt water section. Photo by Laszlo Cseh

As the upstream dives extended to the northwest, the cave started to drop below halocline level, which in this area isn’t a good sign. The tunnels were amazingly beautiful, full of decorations and deep blue water, but as we continued northwest, the flow seemed to stop and the tunnels narrowed. Luckily, one freshwater lead remained to the northeast—a difficult one, due to its small size and the amount of sediment—but, after navigating it and passing a smaller tannic room, the cave opened up again. This side of the cave led to another bypass area, then to the previously discovered saltwater section, which allowed passage for more gear in order to get further. 

Penetrating further north from the new lines, we found irregular and small freshwater tunnels, but farther upstream they all hit a massive collapse. One tunnel at halocline level continued past a sidemount restriction, but this one also ended, dropping down into the saltwater.

As we were wrapping up the final leads on the new upstream area, the total new distance explored stood at 10 km/6 miles. One remaining lead dropped back into the saltwater, but this time the cave looked different. The walls of the cave were much darker. Light brown, easily disturbed sediment covered everything. Passing through a new sidemount restriction, the cave opened up into a very defined and wide tunnel leading to one of the biggest rooms we have ever seen underground in a wet cave in Mexico. Some parts of it were in the freshwater and some still down in the saltwater, and it was highly decorated with cave formations. 

It’s About the Connections

The saltwater tunnel leading to the room had some animal bone remains, most likely belonging to some kind of bird. On the other side of the room, we dropped down into a decent-sized tunnel that started to turn downstream. We reviewed our survey upon returning home, deciding that traveling downstream from our main entrance could possibly provide a connection to new big room and, if we were lucky enough to find a new route, it could be shorter (1.5 km/0.9 miles instead of 2 km/1.25 miles) and potentially easier than our current route. During two dives from downstream Yax Chen, we found the end of line from our previous exploration and the team discovered two new cenotes along the way. We named them Yax Ich which means “green eye,” and the other K’aas Naay, or “bad dream.” This established one upstream and one downstream connection of the Yax Chen area. Fortunately, this connection proved to be easier to travel and more leads were waiting for us. We found old exploration lines within about 500 m/0.3 miles of our newly discovered big room.

For quite some time, no one had made new connections between the historic area of Ox Bel Ha and Yax Chen, so we have decided to travel northeast to determine if we can make it all the way to the historic Ox Bel Ha. The dives turned out to be maze-like and heavily silty, but we dived two nicely decorated, complex freshwater tunnels, where one ended in a tannic collapse and the other had a challenging sidemount tunnel which we named the “Wormhole.” Although this tunnel was well defined, it still had enough small holes to make it difficult to choose which one to peek into, but the constant flow justified the effort. It took some dives to take the line further, but the cave finally opened up again. 

Emőke swimming towards the ‘Wormhole’ tunnel. Photo by Laszlo Cseh

From this point, we were sure we could connect to Ox Bel Ha, since the big open tunnels pointed straight toward the historic section (which perhaps hadn’t been visited since 1999). Some of the tunnels had traces of charcoal and remains of unidentified animal bones. After some struggles, we made our first connection through a very difficult, low-ceilinged, muddy tunnel. Later, just by accident, we bumped into the old line five more times as we explored leads to the north. As we started to get boxed in by the historic lines, and acquiesced to the salt water that “killed” our tunnels on the west side, we realized that all good things—this amazing, laborious experience included—must come to an end.

To wrap up the remaining leads, we organized three consecutive days of exploration, bringing our project’s total underwater time to 21 hours. Almost the entire exploration took place underneath a swampy wetland area and, as a result, some parts of the cave were often filled with tannic water. We named this new section of the cave “The Swampland” or “Los Pantanos” in Spanish. 

Today’s generation of explorers needs to examine the environment differently than the cave’s original trailblazers. The standardized methods of team diving and the excellent training we received from GUE helped us overcome these difficulties, but without access to high-quality gear, this project wouldn’t have been possible. Fortunately, we were able to alternate between a back mounted and sidemount tank configurations as needed and to develop systematic methods of transporting a lot of equipment even further into the cave.

We would like to express our thanks to Dr. Mario Valotta for his support with our dive gear; a very special thanks to GUE instructor trainer Daniel Riordan for joining us for one very productive day of exploration.

Additional resources:

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

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


Emőke Wagner is originally from Hungary and began diving at a young age. She has been an active instructor since 2014. After a couple of years spent traveling around the globe, she moved to Mexico with her husband in 2017. While living in Mexico, cave diving became her real passion, and she began exploring more of the local cave systems. Since 2016, Emőke has been working as a full-time GUE instructor and is currently teaching the cave, foundational, and recreational curriculum.

László Cseh is from Hungary and has always been fascinated with the underwater world. He became a recreational diving instructor in 2012 and began teaching and traveling with his wife, Emőke. After becoming a GUE instructor in 2016, he moved to Mexico to look for new diving challenges. Local cave exploration possibilities helped him achieve his GUE Cave instructor certification.

Bjarne Knudsen began diving in 1993, taking his first tech classes in 1997 and his first GUE Cave and Tech classes in 1999, so has been part of the GUE community since the early days. In the early 2000s, he spent some years in Florida, where he was a part of the WKPP. During this time, he also pushed Sheck Exley’s end of line in the Cathedral Cave system with Todd Leonard (and lots of support from friends). Bjarne is currently on a slow world cruise with his wife on their sailboat. For the last few years, they’ve been a little stuck in Mexico and the surrounding countries, which offer so much nice diving.

Categories: Exploration, Latest Features, Cave Diving, 

Tags: V 3.6, Cave diving, Ox Bel Ha, GUE project, Exploration 

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Karen van den Oever Continues to Push the Depth at Bushmansgat: Her New Record—246m

Karen van den Oever recently broke her own world cave diving depth record by a little more than 10m/33 ft at Bushmansgat cave in South Africa. The S.African cave diver conducted the 8 hour 14 min high-altitude dive on open circuit scuba, breathing trimix 4/90 bottom mix, and suffered mild High Pressure Nervous Syndrome (HPNS). Here former world depth record holder, Nuno Gomes who was van den Oever’s cave instructor, offers the details of her record setting dive along with a short history of the women’s depth records.

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By Nuno Gomes. Images courtesy of Karen van den Ever.

Karen van den Oever and her husband Francois Bain

Karen van den Oever, from Johannesburg, South Africa, has dived to a depth of 246.65 m/809 ft. This is equivalent to a dive to a depth of 296 m/971 ft when corrected for an altitude of 1550 m/5,085 ft above sea level. The dive was conducted on October 27, 2022, in Bushmansgat cave, South Africa, and is a new women’s world record cave dive. Karen bettered her own previous world record to a depth of 236.04m/770 ft  (283 m/924 ft correcting for altitude), also accomplished at Bushmansgat cave in 2021.

I actually felt really good after the dive, a little tired but overall, quite good. I felt much better after this dive than the previous one. I’m happy that the dive went well, just thinking about what comes next. I have no definite plans going forward, we are looking into diving some of the caves in Namibia and also exploring some of the caves not yet dived in Zambia but no concrete plans yet.”—Karen van den Oever

Karen and Theo van Eeden, with the signed tag.

Women have been making record deep dives for quite some time. Back in 1981, one of the first deep diving records was made by Sheck Exley’s wife, Mary Ellen Eckhoff (USA). She used a dive propulsion vehicle (DPV) to travel into Wakulla Springs cave, as well as staged tanks for decompression purposes. Mary Ellen dived on open circuit, together with Paul DeLoach and John Zumrick, and they reached a distance of 363 m/1192 ft and a depth of 80 m/260 ft, which was a major dive at the time.

In 1996, Dr. Ann Kristovich (USA), a friend of Jim Bowden, considerably extended the record, reaching a depth of 167 m/548 ft on open circuit at Zacaton cave, Mexico. Ann’s world record dive would remain in place for a long time.

It was not until the year 2000 that another woman, Claudia Serpierri (Italy), would beat the previous record, but this time in the sea (Mediterranean Sea). Claudia would reach a depth of 211 m/692 ft on open circuit, diving from a support ship. This dive remains the deepest sea dive by a woman to date.

Toward the end of 2001, Verna van Schaik (South Africa), was ready to challenge the women’s record. First, she did her deepest dive by reaching a depth of  186 m/610 ft  (223 m/732 ft correcting for altitude), on open circuit, at Bushmansgat cave in South Africa. This was not enough for her, and during her next expedition on October 25, 2004, Verna would go back to Bushmansgat cave to become the first South African woman to get her name in the Guinness Book of World Records by reaching a depth of  221 m/725 ft  (265 m/870 ft altitude corrected), on open circuit. Her deep support diver was the late Dave Shaw (Australia), on closed circuit, who died of respiratory insufficiency at a sub-250 m dive at Bushmansgat in 2005.

View of the surface pool of Boesmansgat cave.

Following Verna van Schaik’s dive at Bushmansgat cave, two women divers died trying to break her record, as follows: 

In May 2010, French diver Brigitte Lenoir, died in Dahab, Egypt during a dive in the Red Sea. The accident took place at 147 m/482 ft while ascending from a 200 m/656 ft, on closed circuit. Her body was recovered with an ROV. 

In September 2017, Bulgarian technical diving instructor trainer, Teodora Balabanova, died attempting a dive to 231 m/754 ft, on open circuit, while her husband, Mihail Balabanov, suffered from decompression sickness. 

Karen van den Oever is a science graduate from the University of Witwatersrand in Johannesburg, South Africa, where she currently resides. Like Verna van Schaik, who now resides in New Zealand, she is a CMAS diving instructor, and also a former member of the University of the Witwatersrand Underwater Club. 

Her original cave, trimix and blending training was with me. I also trained her husband Francois Bain. 

Unstoppable Karen van den Oever

Karen had previously dived to 201 m/660 ft (241 m/792 ft altitude corrected) on open circuit in Bushmansgat cave in South Africa’s Northern Cape province on February 27, 2020. That dive’s total dive time was 7 hours and 21 minutes. On March 26, 2021, Karen dove to 236.04 m/770 ft (283 m/924 ft), on open circuit, at Bushmansgat cave, using a bottom gas of trimix 6/85. The total dive time was 7 hours and 18 minutes. That dive is the current deep diving Guinness World Record (women).

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Karen’s new world record dive, done on October 27, 2022, was made to a depth of 246.65 m/809 ft (296 m/971 ft), in Bushmansgat cave. The dive was done on open circuit, using a bottom gas of trimix 4/90, and with a total dive time of 8 hours and 14 minutes. The dive would not have been possible without a large team of support divers. 

Karen’s dive computer. Actual depth from rope measurements by independent witnesses was 246.56m/809 ft.

Peter Reid was at 209 m/686 ft (251 m/823 ft); this was his personal deepest dive on closed circuit, and his total dive time was 6 hours and 20 minutes. Don Hauman did deep support at 110 m/361 ft (132 m/433 ft). Her husband Francois provided shallow support and surface support, together with the other team members.

Karen’s support team.

Karen’s Total Narcotic Depth (TND) was 48.06 m/158 ft; the Equivalent Narcotic Depth (END) considering nitrogen only was 9.49 m/31.14 ft, and her maximum Partial Pressure of Oxygen (PO2) was 1.03 Atm. Gradient factors: 40/75.

There were no serious incidents during the dive except that Karen suffered some mild High Pressure Nervous Syndrome (HPNS), which ultimately did not prevent her from going any deeper. Karen had some difficulties recovering the evidence tag from her maximum depth because of the tremors that she was experiencing as a result of the HPNS, but in the end she turned the dive mainly because she ran out of bottom time. 

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Dive Deeper

InDEPTH: South African Cave Diver Karen van den Oever Sets New Women’s Deep Cave Diving Record

InDEPTH: Diving Beyond 250 Meters: The Deepest Cave Dives Today Compared to the Nineties

InDEPTH: Extending The Envelope Revisited: The 30 Deepest Tech Shipwreck Dives

InDEPTH: Opinion: Don’t Break That Record

InDEPTH: Fact or Fiction? Revisiting Guinness World Record Deepest Scuba Dive


Nuno Gomes is a professional civil engineer, a CMAS technical diving instructor and a commercial diver. He was born in Lisbon, but his family relocated to South Africa during his youth. He now lives permanently in New York with his family. He has dived all over the world.

He used SCUBA (open circuit) to dive to a depth of 321.81 meters (1,056 feet), inclusive of rope stretch, in the Red Sea off the coast of Egypt near Dahab, in June 2005. The total dive time was 12 hours and 20 minutes. The descent took 14 minutes with two minutes spent at the bottom.

He also used SCUBA (open circuit) to dive to 282.6 meters (927 feet) in the Bushmansgat cave, in South Africa, in 1996. The cave is located at an altitude of 1,550 meters (5,086 feet) above sea level, which resulted in a decompression schedule for an equivalent sea level dive to a depth of 339 meters (1,112 feet) in order to prevent decompression sickness. The total dive time was 12 hours and 15 minutes with four minutes spent at the bottom of the cave.

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