Early Exploration

 In 1984, a team of cave explorers led by Dr. Bill Stone of the U.S. Deep Caving Team explored a remote resurgence cave (the Cueva de la Peña Colorada) in the Mexican state of Oaxaca. Over three months, Dr Stone’s team explored five kilometres into the mountain, requiring them to tackle several sumps – a sump is a fully submerged part of a cave that can only be explored with dive gear – and established the first subterranean camps beyond a sump (this would be a world first). Over several weeks, the team explored deeper underground before finally stopping at Sump 7.

Sump 7 lay at the bottom of an underground 55m vertical drop. With nowhere to put on diving gear below, the explorers had to descend this vertical drop with all their diving gear worn. Nevertheless, the team still descended into the sump, a completed several exploration dives. However, with the sump more 50m underwater, and the limitations of diving equipment of the day, the explorers hit the limits of diving at depth and were forced to return to the surface.

The team reported seeing a large passage continuing north underwater, leading them to believe that this cave might connect to another system high on the mountain. However, new technology would be required if further exploration of these deep underwater cave systems were to be possible. Although Dr Stone would never return to Peña Colorada, he did go onto develop the CIS-Lunar rebreather – a complex piece of diving kit that recycles oxygen from the diver, including removing CO2 – and which now allows new teams of explorers to return to the cave system.

A Plan to Return

Andreas Klocker, an oceanographer based in Hobart and fellow diver Zeb Lilly, planned to return to Sump 7 and continue exploring beyond the sump to determine whether the cave system would eventually connect end to end. This would be the first team to return to Sump 7 since the first exploration in 1984.

Klocker and Lilly had both explored other areas of the cave system in previous years and would be joining other veterans of the area, in UK cave divers Chris Jewell and Connor Roe. It was thought that perhaps Peña Colorada and Sump 7 were overflow systems within the cave and only flowed during the wet season. During the dry season, it was hoped that some of the sumps would be dry and further exploration possible.

A Logistical Challenge

Unlike Klocker’s previous explorations, involving small teams of divers and equipment, this latest expedition would require all their to be transported through a complex cave system with multiple ascents and descents, wet and dry sections and require the team to spend up to a week completely underground. A team of 24 cavers and support crew were brought together to make this journey possible. The entire expedition took a year of planning, writing grants, finding sponsors, preparing gear and getting the entire team organised. In the end three tons of gear was transported to Mexico, to support a very small team of divers.

The Gear Haul

 To access the cave, the team needed to descend 700 metres into the Pena Colorada canyon, before transporting all the gear five kilometres underground, including five sumps (one sump could be bypassed) before ending up at Camp 2, above Sump 7.

Sump 2 was very short – although just a little too long to dive it holding your breath. Hauling gear over a huge pile of boulders you reach Sump 3, which had poor visibility and required several dives just to line (string chord) the passageway. On the far side of Sump 3, a 15 metre steep upward climb required ropes to be rigged. Beyond this an underwater lake (the Grande Lagoon) marked the first underground camp (Camp 1).

While relining Sump 4 and 5, remnants from the 1984 expedition were still visible and aided in the divers being able to quickly identify the entrance and exits of these two sumps.  After a long traverse Camp 2 was established above Sump 7 (note: Sump 5 and 6 are the same given Sump 1 was now bypassed).  While the forward team were establishing the route deep in the cave, the support crew were lugging gear from Sump 1 all the way to Sump 7.  It took almost a week just to haul all the required gear from the cave entrance to Sump 7, including setting up diving platforms for the descent into Sump 7.

Sump 7 – First Dive

Chris Jewell describes the first dive into Sump 7:

With Connor laying the line, I followed him across to the far wall of the spacious shaft. With visibility around five metres it should have been easy to navigate but the scale of the underwater cave still meant our lights often failed to reach the walls. After picking our way through a boulder choke we descended to 33 metres and entered a large horizontal passage which matched the description of the previous explorers.

Following this passage steadily down we noted several alternative passages but instinctively we headed deeper along the largest tunnel. At the lip of a small pot we peered down into the depths before descending to a rock and gravel floor at 51.5m depth. A little further ahead the large passage stopped abruptly in a pile of well consolidated boulders jammed against each other.

We couldn’t believe it – the cave couldn’t end like this, surely we had missed something! Retracing our steps while decompressing we scoured the walls for the missing way on but nothing could be found except the previously noted side passages which we decided to leave for the next day.

This was bad news for the expedition. After several more days of diving, the long-hoped for connection between the Cueva de la Pena Colorada and Sistema Huautla suddenly fell to pieces. The team returned to the surface to consider their strategy moving forward. It was agreed that a week later Klocker and Zeb Lilly would return to Sump 7 and see whether Chris and Connor had missed anything.

Sump 7 – Second Attempt

As Klocker and Lilly worked their way back toward Sump 7, heavy rain began to fall outside. This far underground, there was no way to communicate with the team above ground and the cavers had no idea about the change in weather. Nevertheless, Klocker and Lilly were able to return to the underwater pile of boulders and despite poor visibility (~7 metres), the pair were not able to find a way around the boulders and returned to the surface of the sump to decompress. After a few hours on the surface, Klocker returned solo for one final search, which included searching several other connecting passageways. Unfortunately, Sump 7 was a dead end.


After eight days underground, a team of six were still in the cave system, slowly moving gear between camps. Klocker describes what occurred next:

I was lugging gear in an area known as the Grand Lagoon deep inside the system when the silence was suddenly broken by a deafening noise as if giant water turbines had suddenly been turned on. The only explanation for the noise was that an unexpected downpour was rapidly filling the cave system we had been exploring. I quickly ran to the others. Zeb was ascending a rope above a vertical drop. He told me that the water level in Sump 3 had just come up by a metre. With the thundering noise at the Grand Lagoon and the fast-rising water in Sump 3, it was clear that we were in big trouble.                                   

I must have had a worried look on my face. Zeb, who has probably spent more time with me in remote cave passages than anyone else, realised immediately that this was serious. The six remaining cavers decided to make a dash to safety further into the cave in an area known as the Whacking Great Chamber. If we could get there we would be safe from drowning, however the water was rising fast and in the narrow passage leading to the chamber we had just 10 centimeters of clearance beneath the rock ceiling above.

Bivy 69

The group reached Whacking Great Chamber, however were still more than a kilometre from the exit, and flooded parts of the cave separated them from Camp 1 and all the dive gear which was above Sump 3. Gilly Elor describes the time spent in-between the two flooded passages that trapped the team in the cave for the next 69 hours.                                            

We knew that in the Great Whacking Chamber we were safe from drowning, but as the water level continued to rise we began to speculate as to how long it would take to drop…or would it even drop at all? Could the high-water level be the more normal state of The Cueva de la Peña Colorada? What if it rained again?

This isn’t a cold cave, and we all had wetsuits and a space blanket. Hypothermia was unlikely. However, the only food we had was four granola bars between the six of us. The only action we could take once trapped was to lie still in the dark, conserving energy and headlamp batteries while attempting to keep warm, and hope that the water level would drop — which we knew was our only way out.

That first night, as the water level continued to rise, nobody talked. What would we have talked about? Our outside lives. I think we were all contemplating the possibility that we may not get out. Eventually the water level began to slowly drop. We continued to huddle in the dark, listening to the sound of the gurgling water and coming up with theories justifying why every noise was a good sign.

As time passed we also grew weaker from lack of food. After 48 hours we split two out of the four bars six ways.”


During this time, Teddy Garlock, one of our support divers, was pushing into the cave to rescue the stranded cavers and had reached Loma Grande. At this time nobody on the surface knew exactly what was going on in the cave.

Teddy recalls:

When I arrived at Sump 3 I was a little taken back by all of the Nalgene water bottles floating in the sump pool. Then I saw Tomasz’s rebreather floating amongst the debris. Shit! Big problem.

My immediate concern was the dive guideline – lost somewhere in the flooded sump pool. My first attempt at finding the line involved wading in the water with just a mask and a light, hoping it was close. I soon kitted up with my tanks and began a lost-line search. My 25m of line was quickly exhausted as I swept from side to side, finding dive gear strewn across the bottom but no guideline. Using a second reel which I found in the submerged gear, I finally found the guideline and tied in.

After recovering all of the floating and most of the submerged gear I filled a dry tube with water bottles and set off alone into sump 3. I arrived at the primary tie off on the far side of sump 3 which was also under a significant amount of water. Having used all of the spools I carried and still not finding the guideline I had no choice but to turn around and return to surface, otherwise risk losing the guideline I’d laid altogether. I left a dry tube with food and batteries tied off on the line.      

After waiting two days I returned to Sump 3 and noted the water had receded around eight metre. I made the dive alone and extended the guideline on the far side, surfacing in a huge air chamber with 6 headlamps staring back at me. Mirek was the first one I spoke with and he gave me a brief synopsis of their experience: “Three days we’ve been stuck in an air chamber, no food and only one space blanket. We’re getting out right now”. With that Mirek disappeared into Sump 3 and began making his escape

The Salvage Operation

After a week of waiting and resting, with the floodwaters subsided, the team returned to the cave to recover the remaining gear. The cave between the Whacking Great Chamber and Camp 1 was still flooded, and required another section that now required diving equipment.

When the team arrived at Camp 1 all their gear except one light and camera was gone. The force of the water during the flood had washed all the gear down an unknown passage or had simply buried the gear beneath sediment. Some gear was recovered at Sump 4, although the flood had clearly wreaked havoc underground and the entire team considered their luck in having survived the underground flood.

The Future

Andreas Klocker gives his thoughts on this latest expedition:

We learned a lot from this expedition. Sump 7 in the Cueva de la Peña Colorada was a dead end. This was just bad news for our expedition. It had been 34 years since this section of cave had first been explored and for several years our team had been planning and preparing for this expedition. We had high hopes to push through this sump and connect into the underground river that would connect us to the Sistema Huautla, and the Huautla Resurgence and eventually exiting back out the other end where the underground river re-emerges in the Santo Domingo Canyon.

We will now need to return to the Sistema Huautla and approach the cave system from the other side. This will require a dive of more than 100 metres, otherwise we will need to find another as yet unknown entrance in the cave system.

It is still unclear what role the Cueva de la Peña Colorada plays in the drainage of the entire system. Fortunately, just before the flood, we were able to pour dye into Sump 7, which later surfaced in two sections of the Santo Domingo canyon. This at least confirms a connection exists somewhere underground. And the flood marks provided further clues on where and how the water moves underground. We solved a small piece of the puzzle, but have many more to go.


This expedition could not have been possible without the valiant effort from the expedition team and the local community of Loma Grande and Mazatlan Villa de Flores. The members of the team included: Zeb Lilly (USA), Andreas Klocker (AUS/AUT), Chris Jewell (UK), Maxwell Fisher (UK), Jim Warny (IRE), Michael Waterworth (UK), Connor Roe (UK), Gareth Davies (UK), Mirek Kopertowski (POL), Adam Haydock (USA), Teddy Garlock (USA), Tomasz Kochanowicz (POL), Matt Jenkinson (UK), Kyle Moschell (USA), Dane Motty (USA), Charlie Roberson (USA), Adam Walker (CAN),  Dave Watts (UK),  Katie Graham (CAN), Matt Vinzant (USA), Josh Brackley (UK), Gilly Elor (ISR/USA), Andrew Atkinson (UK), Fernando Hernandez (MEX), Laura Trowbridge (UK) and Alejandra Mendoza (MEX).

We also want to thank our supporters: KISS Rebreathers, Xdeep, Shearwater Research, Hennessy Hammocks, Nalgene, Light Monkey, Submerge Scooters, Otter Drysuits, Highline Ropes, Scurion, Apeks, OC Lugo, TFM Engineering Australia, DKG Drysuits, Transglobe Expedition Trust, the United States Deep Caving Team, Subsalve USA, Wilderness Lectures, Canmore Cave Tours, The Ghar Parau Foundation, Australian Geographic Society, and the National Speleological Society, and all of our generous donors that helped to make this expedition happen.

For more information have a look at our web page, https://www.beyondthesump.org , and for regular updates on our future expeditions follow us on https://www.facebook.com/CaveDive/.

Photo credits: Adam Haydock (www.even-further.com/), Michael Waterworth, Chris Jewell