WE APPROACH DEVILS HOLE from the north, on a dirt road, skirting a smallish mountain clump that looks as if God accidentally overcooked it. Cynthia looks ahead and says, "There it is." Her landmark is a tall chain-link fence surrounding an acre or so at the foot of the mountain.

After chilling in the Jeep's air conditioning, the skin momentarily lies about the midsummer temperature, but here comes the truth: 110, unmitigated by the pizza-oven breeze. Devils Hole is, indeed, a hole. It brings to mind a flooded mine shaft.

To devotees, though, Devils Hole is a place for wonderment and deep thought. After staring silently for a second, Cynthia says she's thinking about the hydraulics of the hole, a window on an ancient aquifer. Elsewhere the water is trapped thousands of feet down, but here it follows a fault upward. The collapse of a cavern roof about 60,000 years ago opened up the hole, whose depth is unknown. So far, divers have gone down 460 feet without finding the bottom. Theories vary about when and how fish got in, but there's general agreement that it happened roughly 10,000 years ago, at a time when this part of the world was still wet. Some scientists entertain a theory that genetic exhaustion—cousins marrying cousins for millennia—could be the real species killer here.

Cynthia also muses about one of the hole's spookier phenomena: sloshing brought on by distant earthquakes. During an earlier visit I made to Devils Hole, Death Valley park biologist Linda Manning told me that the 2002 magnitude-7.9 Denali earthquake, in Alaska, caused the hole's water level to rise and fall about six feet. Lunar gravitation also moves the water higher and lower with the same twice-a-day rhythms as ocean tides. Manning let me go down to the water's edge, where three inches of wetness marked a drop from high tide to low.

The most practical down-hole path leads to the edge of a shallow shelf, about 17 feet long and six feet wide, with balls of fuzzy algae and a few packs of fish chasing around. Though they swim down to 80 feet or so, pupfish pass most of their time up on the shelf—hatching, growing, spawning, feasting. No shelf, no pupfish, the savants say. And the margin of survival, in terms of water level, is small. Mike Bower, Death Valley's fishery biologist, says a drop of six inches would be big trouble and adds, "A foot could be the coup de grace."