The temperature is starting its climb towards 123 degrees when I catch up with Sada at Death Valley National Park. As the raven flies, Death Valley is around 225 miles from Big Springs. It's a world apart on the surface, but the geology and the hydrology are surprisingly similar. Sada, a trim, athletic-looking man with short-cropped gray hair, is dressed in his work clothes: shorts, T-shirt, a wide-brimmed hat and Teva sandals. "Let's go!" he says. We stop first at Badwater Basin, which, at 282 feet below sea level, qualifies as the lowest spot in North America. There we walk around the rim of a pool whose water shines like burnished copper. Save for floating mats of algae and a border of salt-tolerant pickleweed, it seems devoid of life. 

Then Sada removes a chunk of salt-encrusted travertine and flips it over. On the moist underside, a freckling of snails clings to the rock. On a whim, I touch the water. At just over 70 degrees F, it seems cool compared to the air, which feels like it's been roasted in a blast furnace. At Sada's suggestion, I bring a drop to my lip. While not as salty as seawater, it's definitely brackish, which explains how Badwater got its name. "I still don't know how these things do it," Sada says of the snails. "For me, finding these snails here, in this harsh environment, is just a humbling experience."

The Badwater snail was first described in 1948. It provided one of the first hints of the arcane world that Sada and his longtime collaborator, Smithsonian Institution zoologist Robert Hershler, went on to discover some four decades later, when they set out to look for springsnails in several hundred Great Basin springs. "We didn't know what we would find," remembers Sada. "We ended up with a whole new fauna." In less than 10 years, the number of known North American springsnail species more than doubled. Among the additions was Snake Valley's Pyrgulopsis anguina, whose name combines pyrgos, Greek for tower, with anguinus, Latin for snake-like.

Many springsnails are confined to extremely small habitats. At Badwater they occupy the rocky strip at the water's edge, close to the source of the spring, where the freshest, coolest water wells out. In places this biotic zone is no more than 6 inches wide. Visitors to Badwater were unwittingly trampling it, Sada says, until the National Park Service protected the site with an elevated viewing platform.

Working our way up in biological complexity, we continue on to Travertine Springs, whose water supplies the Death Valley resort of Furnace Creek. There we walk alongside a fast-flowing brook clogged with non-native palms that have taken root right in the water. Even so, the brook continues to host an assemblage of endemic invertebrates -- a springsnail, two amphipods (which are small crustaceans) plus a beetle and a bug. About the size of a flattened pea, the bug, a naucorid or creeping water bug, is the largest predator in the system. "Welcome to my world, the world of minutiae," Sada says.

The day has all but dissolved in a heat-shocked haze by the time we head across the Nevada state line, arriving mid-afternoon in the Amargosa Desert, a grim expanse of sand dunes, alkali flats and sun-scoured scrublands. In this drear setting, the deep aquamarine pools we find at the Ash Meadows National Wildlife Refuge seem more exquisite than gemstones. Among the pools' inhabitants are pupfish, which dart to and fro, nipping at tasty diatoms and small aquatic invertebrates. The females of this endangered minnow species are green; the males, vivid blue, so blue they would appear at home swimming round a tropical reef.

Like the pupfish, the water is always in motion. Where it bursts from the rock, it stirs up puffs of sediment and causes strands of filamentous algae to sway, as if in a gentle breeze. In spots you can even see the plumbing -- the limestone caves from which the water streams. Sada recalls diving down to the cave at the bottom of Crystal Pool to search for springsnails. "The water was coming out with such force, it was hard to swim against it," he says.

The pools of Ash Meadows seem idyllic, until Sada points out some of the non-native intruders -- the school of mosquitofish swimming near the surface, a diaphanous mass of bullfrog eggs. The mosquitofish were no doubt introduced for mosquito control, the bullfrogs probably for food. "People like to move bullfrogs around," Sada says, "probably because they like to catch 'em and eat 'em." Unfortunately, mosquitofish are extremely aggressive, harrassing other fish and eating their offspring. And bullfrogs are not harmless, either: Not far from here, Sada collected one that had a half-dozen partially digested pupfish in its gut.