Mexico’s Sistema Huautla is perhaps one of the most complex of the world’s deep caves. With 17 entrances and numerous independent and physically demanding deep routes, dropping nearly a mile into the earth, this dark labyrinth requires extensive rope work and multiple days of effort to reach the lower depths of the system. The last extension to the cave took place in 1994, when Dr William Stone used his self-designed CIS Lunar rebreather to pass what was then considered the terminal sump. Staging their dive from a portable platform suspended over a pool of water, the transiting a flooded tunnel and went on to discovered some 3.3km (2 miles) of new passage, but were ultimately stopped by Sump 9.
In 2013 a multinational team of cavers and divers once again visited Sistema Huautla in hopes of pushing deeper into the flooded lower passages. Rebreathers would be key to performing these dives, due to the difficulty of hauling equipment deep into the cave. In the years since Stone’s 1994 expedition, rebeather diving has developed significantly, both in terms of technology and knowledge. In contrast to the complex electronic rebreathers used previously, divers in the 2013 expedition all used manually controlled rebreathers fitted with KISS valves.
Caves present one of the harshest and least forgiving environments on earth. Mud, sand, grit and constant damp all create problems for complex electronics, and the remoteness of a cave such as Sistema Huautla magnifies the need for self sufficiency and the ability to solve problems on site. As a result, the most important features of any rebreather used at the bottom of a remote and deep cave are reliability and simplicity.
Lead diver Jason Mallinson relied on a KISS rebreather system to access virgin passages and set a new cave depth record. This is the story of his dives, told in his own words.
“It was Sunday evening when we finally heard the roar of the final cascade. We were following Tim Allen who was rigging the abseil [rapelling] ropes, and had arrived at Sump 1 in Sotano de San Augustin, the main sump in the Sistema Huatla cave system. In 1994, the last time this sump had been dived, both tragedy and success had been experienced by the explorers. [At that time, expedition member Ian Rolland perished on an exploratory dive; Bill Stone later pushed on to discover a major section of the cave.
“It had taken more than a year of planning to get the expedition off the ground, but just one week to rig the cave with ropes from the San Augustin entrance, around 850m above the sump. Tim, Pete, Martin and Mark had been slowly rigging the Lower Gorge beneath Camp 3 and I had been following behind with the KISS Classic rebreather, eager to begin the diving phase of the expedition.
“The KISS had been stripped down to bare basics for the trip down the cave, weighing in at just 8kg (17.5 pounds) with all the hoses, oxygen delivery system and oxygen display units removed and packed in separate waterproof containers. All the ports had been made water tight for the several swims that would be encountered.
“Sump1 has no ledges or dry areas for staging. To provide a staging point, our team rigged a suspended platform over the sump. During this process, the KISS was balanced on a rock just above the swirling water, waiting to begin assembled for use. Not an ideal situation, but the best resting place that could be found before the platform was constructed.
The Kiss was reassembled. The oxygen delivery system was plumbed back in and the breathing hoses were replaced once the blanking plugs had been removed. For cave dive use, I never carry an onboard diluent, as we need large volumes of bail-out gas. All diluent is plumbed in from an off-board source—in this case two 9-litre composite cylinders. The most precarious part of the building process was filling the stack with sofnolime while trying to balance it on a rock situated just above water level. Once everything was made ready, the cells were calibrated and the unit was trimmed with lead and made dive ready.
“The first dive in Sump 1 was dedicated to relining the passage, as it had been 20 years since anyone had been there, and the original line would probably be trashed. The dive began in murky visibility, and once I had dropped down from the initial shallow section, route finding became difficult in the large, silty passage. Remnants of debris gave an indication of the route forward, and once the deeper section (-25m) was passed, the walls narrowed and the route was easy to follow along one wall.
“The Rolland air bell was reached in about 30 minutes. The large sandbar described by previous explorers was no longer present, leaving an easy swim along the surface to Sump2. Here, the old line was found and a new guideline installed alongside it. The KISS had been performing flawlessly throughout the dive and there was no hesitation in continuing through to the dry passages discovered in 1994.
“Upon surfacing, the oxygen delivery system was turned off and the KISS deposited on a convenient rock, while a short reconnaissance was made along the large borehole. About 45 minutes after surfacing, I headed back through Sump1, this time a bit faster as I was confident that the guideline was now suitable for the other follow divers to follow.
“An additional week of hard work was required to transport all the dive equipment for the four other divers down to Sump1, and to ready the specially made dry-tubes that would carry the camping equipment. The plan was for five divers to camp beyond the sump for seven days.
“One week later, after spending our first night beyond Sump 1, we were ready to enter Sump 9. The entrance pool was large and foreboding, surrounded by steeply descending muddy walls. It had been a personal ambition of mine to dive this sump for many years, and the plan to organise an expedition had been hatched in 2008, immediately following another deep cave diving expedition. Now here we were, with the tools to do the job.”
Over the course of five dives, Jason Mallinson and Chris Jewell explored sump 9 in poor visibility conditions. After the fifth dive the underwater tunnel had been extended to a depth of 60m at a point 300m into the sump.
“On the final day of diving, I was allocated the first dive, and Chris would follow if the sump was still going at a reasonable depth. I towed an extra Trimix cylinder to a penetration of 200m (650 feet) and left it clipped to the line resting on a rock, then swan on to the limit of penetration at around 300m (980 feet) into the sump. Tying on the line, I checked all was OK with the KISS and as usual there were no problems. The roof continued to dip. The dive computer was soon showing a depth of 75m (246 feet) was showing on the dive computer and the roof was still heading down. I decided that 80m (262 feet) would be my limit, as I had already accumulated significant decompression. Soon enough I hit -81m (265 feet) depth and tied off the line. At this deep point, we were -1545m (5062 feet) deep in Sistema Huautla and had taken the Western Hemisphere cave depth record back from Cueva Cheve (also one of our dives). With no time to consider the implications of the depth, I headed out as quickly as possible while taking notes for the master survey of the system. A 90-minute decompression brought the total dive time to 150 minutes, all carried out flawlessly using the KISS rebreather.”