Three weeks into their expedition, Chris Jewell and his team arrived at what looked like a calm, rock-enclosed lake about 100 feet wide. But as they knew, this was no lake: It was a sump, a low-lying trench where water collects – and they were almost a mile beneath the surface of the Earth. "When our toes were at the water's edge, we knew we were in new territory," says Jewell. "You're going somewhere no one else has been before."

Since its discovery in 1965, Sistema Huautla, a cave system tucked into Mexico's Sierra Mazateca mountains, has lured explorers from around the world. "If Dr. Seuss had done a book on caves, it would look like this," says veteran caver Bill Steele, 65, who has taken over a dozen trips to Huautla since the late '70s and joined in for part of this year's trip. Thanks to its seemingly endless passages, waterfalls up to 60 stories high, and natural wonders like the massive, 300-foot-high chamber called Anthrodite Hall, it is, Steele says, "probably the best cave on earth - lots and lots of variety."

This year's expedition, organized by Jewell, a 30-year-old software consultant and caving enthusiast from England, had one objective: to go beyond that trench – called Sump 9, discovered in 1994 – and set a new record for the greatest depth ever reached in the Western Hemisphere. The only option? Dive into the murky water to see where it would take them. "We knew there was an opportunity to look for new passages," says Jewell. "Most people reach a sump and think it's time to go home. We were just getting started."

The seven-week trip began in late February. The 40-person international team slogged more than a thousand pounds of gear, including rope, wetsuits, and headlamps, into the cave. They spent two weeks rappelling to an underground camp about a half-mile into the Earth, navigating some of Huautla's 20 waterfalls as well as passages deluged with waste-filled water surging through the mountain from villages above. The cavers occasionally surfaced for cooked meals and rest, but the record-breaking portion of the journey began when five divers, including Jewell and fellow Brit Jason Mallinson, spent 10 full days underground exploring Sump 9. After locating an underwater passage, Mallinson plunged in, traveling a distance of 265 feet. With that dive, he reached a total depth of 5,069 feet, officially the deepest known point in the hemisphere.

"Things like Everest you can scout from a plane with binoculars," says Jon Lillestolen, 33, an American member of the squad. "With caving, you don't know what's around the next corner. Huautla definitely lives up to its legendary stature. It's just monstrous."