Devil's Hole, where the earth opens to reveal a rare window into Death Valley's most implausible underworld, a universe of water. In twisted irony, the driest place in the hemisphere sits atop one of the largest aquifer systems in the United States, spanning nearly 40,000 square miles. And here in Devil's Hole, plunging to unknown depths, geologists once explored the abyss 400 feet and found no sign of bottom. These divers are on a mission in this unlikeliest of desert habitats to save the unlikeliest of desert creatures, and the park's most precious treasure--the tiny Devil's Hole pupfish. Relics of the Ice Age, pupfish once swam at frigid nights, the feet of mastodons. Today, they dwell in this hotspring-fed pool where they thrive for thousands of years until now. With only about 200 left, the species is gravely endangered. Pup biologist Linda Manning and a team closely monitor the fish for clues to their decline.

I think our greatest hope is that this population will continue to survive in this natural environment, the way it has for thousands of years. Our greatest fear is that it will go extinct, and we don't yet understand everything is going on in this system.

Fluctuating water levels, limited food supply, even contamination are possible factors, but it's what they don't know that worries the team the most.