The San Agustín Sump: a kilometre underground and three metres from air

Narrow passages, almost a kilometre underground, black water you can't simply surface from, and claustrophobia so intense my palms start sweating. Exactly my kind of story.

And it isn't about thrill-seekers who can't sit still. There is an engineer who spent years building a special breathing machine for one cave, the death of a teammate, an astonishing discovery and a friendship that turned out to matter more than any record.

A cave that ends in water

Mexico, the Sierra Mazateca mountains in Oaxaca. This is home to Sistema Huautla, one of the deepest cave systems in the world. From the outside its entrance looks almost idyllic: a huge green funnel in thick vegetation. Then come vertical shafts, narrow rock fissures, underground rivers, waterfalls and kilometres of rope.

At the bottom of the system, cavers kept running into the same obstacle for years. The San Agustín sump.

Picture an underground river that suddenly disappears under a rock ceiling. Water fills the tunnel from floor to roof. No air space, no way to lift your head above the surface. And the first rule of diving doesn't apply here: if something goes wrong, go up. Above you is rock. Behind you are hundreds of metres of flooded tunnel. If the water clouds over, you feel your way back holding a thin guideline. Lose it, and you can stay in that maze forever.

The engineer who rewrote the rules

The American engineer and caver Bill Stone realised the problem wasn't just the dive itself. Ordinary scuba needs a lot of tanks. Now try hauling even a dozen heavy cylinders through almost a kilometre of underground shafts, waterfalls and tight squeezes.

Stone approached it like an engineer. If the equipment can't get you through the cave, build new equipment. For years he developed the Cis-Lunar electronic rebreather. Unlike scuba, it doesn't vent exhaled gas as bubbles. A scrubber removes carbon dioxide and a computer adds exactly the oxygen needed. The result is close to a spacecraft life-support system that lets a diver stay underwater for hours while using far less gas.

The 1994 expedition

In January 1994 Stone's team set off for Mexico. The expedition lasted 135 days and listed 44 members, mostly from the United States and Britain. Eleven were divers; the rest hauled gear and ran the camps.

Rigging the route took weeks. People lowered loads on ropes, dragged containers through fissures, ferried equipment past underground waterfalls and set up intermediate camps. Camp 3 went in by mid-March, Camp 5 by 23 March.

This is where I start feeling physically uncomfortable. Camp 5 hung three metres above the black water of the sump. A waterfall roared nearby; around it, darkness, damp and cold. And if someone had to be evacuated injured, the trip to the surface could take days.

The breakthrough

The divers took turns in the sump, pushing a little further each time and laying guideline behind them. In late March, Briton Ian Rolland and American Kenny Broad, diving solo, got through the first sump: after 430 metres of flooded tunnel the passage rose and the diver surfaced in an air chamber about 12 by 50 metres with a sandy beach.

People had tried to get through that sump for years. Now they had.

Three metres

Next came the second sump. Rolland went in alone. He didn't come back.

Exactly what happened was never fully established. Rolland had been diagnosed with diabetes shortly before the expedition. On the expedition site Stone writes that Ian most likely blacked out from hypoglycaemia, a sudden drop in blood sugar, and drowned. His dive computer data showed the rebreather had been working properly.

According to Beyond the Deep, the book Stone and Barbara am Ende later wrote about the expedition, Broad went searching and found his friend's body in shallow water, less than three metres below the surface. He didn't die somewhere in the middle of a long flooded tunnel, lost and out of gas. He was less than three metres from his next breath.

The body had to be brought back through the flooded sump and then raised through the same maze used to lower the equipment. Tight passages, vertical shafts, waterfalls, rope traverses. Only now the team was carrying not gear but a dead friend. The authors recall that people from the nearby village, hearing of the tragedy, came to help.

I really love this part. Foreigners came to their mountains to explore an underground world. When disaster struck, people simply came to help. No records, no scientific goals, no engineering explanations.

The two who went on

After Rolland's death the expedition was close to falling apart. People were exhausted, shaken and unwilling to risk their lives for a few hundred more metres of unknown cave.

Then another piece of news arrived. On 6 April 1994 Sheck Exley, a legend of cave diving, died at Zacatón, a Mexican sinkhole, while trying to go deeper than 300 metres.

Stone decided to keep going. On 8 April he passed the first sump, reached the chamber where Ian had died and dived into the second. After 170 metres the flooded tunnel ended, opening into a dry gallery with an underground river.

For the final big push beyond the sumps only one person went with him: geologist Barbara am Ende. An experienced caver, but with far less experience of hard underwater exploration than the team's main divers. On 1 May the two of them passed both sumps and set up Camp 6 beyond them.

For the next six days they explored places no human had ever been. Enormous chambers, galleries whose ceilings vanished into darkness, underground waterfalls. They mapped 3.3 kilometres of new passage and reached a depth of 1,475 metres below the system's highest entrance. Huautla became the deepest cave in Mexico.

And then they hit a new wall. Another sump, the ninth. In front of it, a chamber almost a hundred metres across; the water itself fifty metres wide.

The black lake

For years people tried to pass the first sump. For even longer they built the equipment to do it. They assembled an international expedition, spent months hauling gear underground, lost a friend, pushed through kilometres of new tunnel. And in the end they ran into the next black lake.

In 2013 a new expedition went back. British divers Jason Mallinson and Chris Jewell made five dives into the ninth sump in poor visibility, pushing about 300 metres in; on the last one Mallinson went down to 81 metres. That single dive lasted 150 minutes, 90 of them spent decompressing. They found no way on.

Today Sistema Huautla has more than a hundred kilometres of surveyed passage and reaches a depth of 1,560 metres. Despite decades of work, cavers still haven't passed all the way through its underwater labyrinth.

The air chamber beyond the first sump bears Ian Rolland's name. The 2013 divers swimming past it called it just that: the Rolland Air Bell.