Diving the underground passages of Mexico's Yucatán sometimes it's easy to forget that there is an unknown world beneath our feet. Take, for example, Mexico's Yucatán Peninsula, where divers Steve Bogaerts, Robbie Schmittner, and Sam Meacham are fast uncovering the longest underwater caves in the world. The action started last January, when Bogaerts, 43, and Schmittner, 33, proved the Sistema Sac Actun to be a record 95 miles (153 kilometers) long. The title did not last, however. In June, Meacham, 40, and his team capped ten years of exploration in an entirely different system, Ox Bel Ha, with the discovery that it was an astounding 102 miles (164 kilometers) in length. To find such caverns, Bogaerts, Schmittner, and Meacham (who work separately but collaborate) troop out to unmapped sinkholes, strap on scuba gear, and descend into the caves—"some so big you could fly a jumbo jet through," Bogaerts says, "some so small we have to take off our tanks just to fit." The results are always unexpected. One time Bogaerts had a passage collapse behind him, while Schmittner once surfaced only to come face-to-face with a jaguar. Each diver has found the bones of woolly mammoths, along with piles of Maya relics; the most extraordinary find was a human skeleton dating back 13,600 years, possibly the oldest in the Western Hemisphere. The beauty of this project is that the discoveries will keep on coming: According to Meacham, "We now think that one day these two cave systems will join."