Sada ticks off a list of other exotics that have invaded Ash Meadows over the years, including crayfish, mollies and an aquarium snail believed to prey on the eggs of fish, springsnails and other aquatic invertebrates. This snail, the Malaysian trumpet snail, is highly resistant to desiccation. It can easily hitch a ride in the beaks of birds or the shoes and clothing of waders. It can also reproduce asexually; one snail is all it takes to start a population explosion.

Less obvious than the exotics are the healed-over scars left by would-be developers, one of whom tried to turn Ash Meadows into a cattle ranch with irrigated forage crops and another -- Jack Soules -- who envisioned building a 20,000-home subdivision. "This is still a deeply traumatized landscape," Sada says, pointing out the row of Native American grinding stones arrayed along a fast-flowing spring brook. "Soules was planning to put his own house right here, with the brook running through his living room," Sada recalls. In the early 1980s, backhoes were already at work here, ripping into mesquite-lined brooks and redirecting their flow into a series of artificial ponds.

"There were dead pupfish all around," remembers Sada, who, at the time, was the U.S. Fish and Wildlife Service's endangered species expert in Nevada. He was one of the Fish and Wildlife Service officials who walked into Soules' office and informed him that he was violating the Endangered Species Act. Construction stopped soon thereafter, rendering further action unnecessary. A short time later, Soules' company, Preferred Equities, sold Ash Meadows to The Nature Conservancy, which, in turn, sold it to the Fish and Wildlife Service in 1984. Today, the refuge protects two-dozen endemic species, including the Ash Meadows Amargosa pupfish and four other species now considered officially endangered.

Before leaving Ash Meadows, Sada takes me to visit nearby Devils Hole, a limestone cavern whose roof collapsed long ago, opening a window to the aquifer below. In 1968, water levels began to plummet here, in response to pumping from irrigation wells sunk into Ash Meadows. Eight years later, a landmark decision by the U.S. Supreme Court effectively shut down the pumping, saving the gravely endangered Devils Hole pupfish. As I stand on a metal viewing platform, suspended high above a pea-green pool, I understand the rationale for the 50-mile road trip we've just taken.  Devils Hole, Ash Meadows and Travertine Springs in Death Valley all tap the same subterranean lode.

Hydrologically speaking, Nevada is a singular place. The driest state in the U.S., it sits at the heart of the Great Basin, an undulating expanse of mountains and valleys that resembles a rumpled carpet. The name stems from a geological insight made by the great pathfinder John Fremont, who recognized that water in this vast region is trapped in the equivalent of a bathtub, "having no connexion whatever with the sea."

The precipitation that falls on this rugged landscape has a couple of options. If it stays on the surface, it may briefly collect in an ephemeral playa or join a stream or a river that empties into a terminal sink or lake. The best-known example of the latter is Utah's Great Salt Lake, whose waters, due to high evaporation rates, are more saline than seawater. Or, it could seep into the ground and end up beneath the surface, in the vast freshwater reservoirs that underlie the valleys of this arid region.

The water in those reservoirs comes from snow and rain in the mountains, which percolates down into near-surface sediments and gravels and seeps beneath them, through porous layers of carbonate rock laid down by ancient seas. For the most part, these subterranean reservoirs are invisible, but here and there, their hidden waters well up through fissures and faults, spilling onto the surface as springs. Early explorers carefully noted the locations of these life-sustaining oases, both within the Great Basin and along its edges. In 1844, for example, when Fremont visited the Las Vegas Valley, he remarked on "two narrow streams of clear water, 4 or 5 feet deep, with a quick current, from two singularly large springs." These springs stopped their year-round flow decades ago, dooming the Las Vegas dace, a native fish, but in Fremont's day they supported a lush growth of vegetation, explaining the name Las Vegas, which is Spanish for "The Meadows."

Both physically and chemically, the springs that draw from the deeper carbonate are different from springs that tap aquifers in the valley or basin-fill. Their waters are older and carry higher concentrations of dissolved minerals. They are also warmer, a sign they have passed through depths heated by the earth's core. At Ash Meadows, for example, the temperature of the larger springs is a balmy 90 degrees. Still, the flow through the carbonate is not completely cut off from the basin-fill. There are zones of convergence. In Death Valley, for example, groundwater is thought by some to cascade through a carbonate spillway beneath the Funeral Mountains, then plunge deep into the basin-fill before resurfacing at Travertine Springs.