Last spring, a Brit-led team created a resurgence of interest in Sistema Huautla, the deepest cave in the Western Hemisphere, originally discovered in 1965. They spent 10 full days underground and reached a new depth of 5,069 feet. On March 28, a 26-person international team led by Americans Bill Steele and Tommy Shifflett, will embark on a four-week expedition to Oaxaca, Mexico to find out just how far down the rabbit hole goes.

A fellow of the National Speleological Society and fellow emeritus of The Explorers Club, Steele has been to Huautla 18 times since he first began exploring the massive cave system in 1977. "It's probably the greatest cave on Earth," he says. "It's already the 8th deepest cave on the planet, and it's longer than the top 16 deepest caves, which means it's huge. And there's so much more we haven't discovered. This is just the tip of the iceberg."

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Steele and Shifflett have devoted the next decade to finding out exactly how mammoth and complex Sistema Huautla actually is. The March 28 trip kicks off a series of month-long expeditions to be held in April every year for the next 10 years. "It's going to take us at least that long," says Steele.

Part of the challenge of understanding Huautla is the varied geologic features: dozens of waterfalls (some 60 stories high), massive chambers like the 300-foot high "Anthodite Hall," and sumps – water-filled trenches resembling underground lakes. "We'll be conducting a full Speleological study – geology, hydrology, paleontology, biology, archeology, and cartography," says Steele.

And then there's the fact that it's underground, with no light, and complicated mazes that must be carefully tracked in order to make it back to the entrance. The team is prepared to spend up to two weeks camping underground to plumb the depths of Huautla. Steele hopes to hit 5,280 feet–the depth of one mile–and to extend the known length from 65 kilometers to 100 kilometers by linking the various underground passageways.