Bonneville's history reflects a global truth. Look at a map of the world and you see that many of the great deserts occupy two latitudinal bands 15 to 35 degrees north and south of the equator. These bands include the Mojave, Sonoran and Chihuahuan deserts of North America; the Sahara, Namibian and Kalahari deserts of Africa; the Saudi and Persian deserts of Asia; and the Simpson and Sandy deserts of Australia. These global desert belts arise from a massive atmospheric conveyor belt called the Hadley cell, which lifts warm, moist air from the equator, and wrings the moisture out as rain or snow over the tropics as the air rises to 40,000 feet. The Hadley cell then dumps the dry air back down to the earth's surface further north and south -- creating deserts with cloudless skies.

The dry beds of Bonneville sit at the northern edge of this arid zone today. But they didn't always."The way you make Lake Bonneville is bring Oregon down here," says Quade. "You've got to bring Oregon down here climatically."

The Laurentide Ice Sheet, a slab of ice several thousand feet thick that would cover modern-day Canada and the Great Lakes, may have done this during the last ice age. The Laurentide diverted a northern belt of wet air called the jet stream southward, away from what is now Oregon and Washington -- pointing it at Utah instead. As that fire hose of moist air hit the 10,000-foot crest of Utah's Wasatch Mountains it dumped snow, forming the glaciers and rivers that fed Lake Bonneville.

Bonneville's heyday may represent the wet extreme of what the Great Basin is capable of experiencing. It is easy to assume that the deserts and arid mountains of today represent the other end of that spectrum -- the dry extreme. But there is reason to think that today's condition is only the middle of that range -- that the West can, and will, become much drier.

Satellite studies suggest that rising temperatures have caused the Hadley cells to widen by 100 to 300 miles over the last 30 years. If this trend continues, the atmosphere's conveyor belts will dump their dry air farther and farther from the equator, shifting the most intense bands of dryness from northern Mexico toward Nevada, Utah and Colorado. Rising temperatures will also reduce precipitation directly by increasing the capacity of air to hold water, says Richard Seager, a climate dynamicist at Lamont-Doherty. "By the middle of the century, models are predicting a 10, 15, 20 percent reduction in precipitation" in the West, he says.

It may sound minor, but in the West, small change carries the day. Lake Bonneville, for instance, may have covered eight to 10 times the area of modern lakes in the area such as the Great Salt Lake, and it should have needed eight to 10 times as much precipitation to keep it alive. But Broecker increasingly believes that Bonneville survived on a supply of rain and snow that was only two or three times as large as that of today.

It comes down to an obscure tenet of watershed hydrology, called the "Budyko curve" after the Russian scientist who developed it in the 1950s. Budyko is a mathematical line that relates the amount of precipitation that falls in an area to the amount of runoff that pulses through its rivers and streams. It is not a straight line; it bends like a boomerang. That bend means something important: It says that the more rain and snow that falls, the greater proportion of it actually percolates into rivers, streams and lakes without evaporating first. This happens because heavier precipitation overwhelms the ability of sunlight to evaporate water -- solar radiation literally can't zap away the water molecules quickly enough if they all arrive at once. It could explain how Lake Bonneville was able to form: Increasing precipitation by only two- or three-fold may well have meant that the lake received eight times as much runoff.

So Budyko turned the Great Basin into an aquatic paradise 30,000 years ago -- a mega version of southern Utah's Lake Powell that Quade wishes he could have visited with a fishing pole -- but it portends a darker vision of the future. It means that as less precipitation falls, the percentage lost to evaporation will rise.

"If there's a 10 percent decrease in rainfall, you get a 30 percent decrease in runoff," says Broecker. "If you decrease rainfall by a factor of two, you would get six times less runoff. The Western U.S., I think, is in for some big trouble." The drought that occurred across the Southwest in the early 2000s provides one possible harbinger of those fears. It browned over 4,000 square miles of juniper-piñon woodland and shriveled reservoirs to record lows before subsiding around 2005.

Even worse droughts are possible. Forty-four years ago, an airplane crashed into Walker Lake in western Nevada, 300 miles southwest of Bonneville. When crews dragged the lake for the plane's wreckage, they encountered a surprise: Their nets caught on the stumps of trees still standing on the lake bottom under many feet of water. Walker is a natural, closed-basin lake much like Bonneville. It is fed by the Walker River, which carries snowmelt from the eastern Sierra Nevada. Those lake-bottom trees could only have grown if conditions were so dry that the lake disappeared.

Scott Stine, a paleoclimatologist at California State University, East Bay, has found similar stands of dead tree stumps up and down the bed of Walker River -- Jeffrey pines, a species incapable of surviving in even a few inches of standing water. These trees are about 900 years old. Their very existence speaks of a drought that was so severe that both the river and the lake virtually ceased to exist for 100 years.

The kind of natural variation that produced this drought, the 1930s Dust Bowl, and last decade's juniper-piñon die-off will continue into the future. But if Seager and Broecker are right, they will occur against a drier backdrop. The droughts will be drier, and so will the wet periods between them.

Lake Bonneville held around 5 trillion tons of water at its peak. As that weight vanished into the atmosphere, the underlying crust of the earth sprang back up like a rebounding couch cushion. Bonneville's bed arched up 200 feet in the center, lifting the Oquirrh Range, the Silver Island Range, and a dozen other mountain chains that it once surrounded. The indentation that Bonneville left in the earth's crust is almost gone now, and other signs are fading, too.

One afternoon, Ali sits on a pile of rubble at the mouth of a cave no larger than a doghouse. Below lies the playa that the Donners crossed 165 years ago, raked by winds that lift clouds of dust hundreds of feet in the air.

Ali and Quade have had a disappointing day. A series of caves and grottoes that must, in the past, have been coated in the minerals they seek turned out to be bare instead -- stripped clean by the elements.

Here at the doghouse cave, Ali and Quade managed to scavenge a handful of rocks -- one of the best finds of the trip, in fact. But far more of the stuff lies strewn down the hillside at Ali's feet -- dislocated, fragmented, useless. The exquisitely layered mineral that they did manage to collect here survived only because it formed in a protected spot when the cave was submerged -- deep in cracks that permeated the cave's ceiling. The bedrock later crumbled away, making the layers visible.

For people seeking to uncover Bonneville's secrets, the target is shifting. The same process that exposed these precious minerals for discovery is also slowly obliterating them. The work of the geologist is to chase these retreating signs even as 99.9 percent of them have already vanished. Ali flicks another rock down the rubbish pile. "This stuff is eroding away," he says. "Ten years from now, this could all have been gone."

Douglas Fox is a science journalist whose stories have appeared in Discover, Popular Mechanics, Scientific American, Esquire, and The Christian Science Monitor.