A curious horizontal line runs across the range -- a notch cut into the mountains like a railroad bed, visible from many miles away. It snakes around every gully and ridge, 600 feet above the playa where the Donners hauled their wagons. Floating Island Mountain, visible to the east above a perpetual mirage, also shows this line. The same thing can be seen across much of Utah, inscribed into every mountain and hill like a celestial constant.

That line records the shores of a massive lake, called Lake Bonneville, which once sprawled across the region. You can spend half a day driving across Bonneville's dusty beds on Interstate 80, beneath hundreds of feet of vanished water, without ever coming up for air. The lake's irregular tendrils stretched for 150 miles east-west and 250 miles north-south; it covered modern-day Salt Lake City and reached across the Nevada and Idaho borders. "This thing used to be the size of Lake Superior," says Jay Quade, a University of Arizona geologist who has spent much of his life exploring these deserts. "It was an inland ocean."

The sagebrush, rabbitbrush and Mormon tea that grow here today subsist on just five inches of rain a year. But as Quade climbs a gully on the west flank of the Silver Island Range, some strange shapes loom into view -- apparitions of a wetter past. In one place, hundreds of stone fingers protrude from the rock like branching coral. Elsewhere, a row of turrets resembling tropical sea sponges clings to the gully's walls, their beige shapes conspicuous against the gray background. Their curves vaguely evoke a living, aquatic origin. The same stuff drapes, petrified and cracking, over the crests of nearby ridges like a six-inch layer of mud.

This spongy stone, called tufa, was laid down by algae. "This whole area was covered in algal soup," says Quade. Today,  it sits 600 to 800 feet above the dusty plains that surround these mountains, but for thousands of years those algal mats basked in sunlight just below the water's surface. The Silver Island Range was truly an island; only its upper slopes rose above the water.

The rise and fall of seas seems like something that could only have happened far back in geologic time -- hundreds of millions of years ago, in a world populated by unrecognizable life forms. But Bonneville's waves lapped against these shores a mere 15,000 years ago. Human beings saw the dwindling lake when they arrived in the Great Basin 1,000 years later. This place has hardly changed since then. You can still see stones as small as a fist, draped in algal tufa, that haven't moved an inch since the inland sea evaporated thousands of years ago.

Scientists have studied Bonneville for decades, often for the pure joy of piecing together a mystery. But these days, the study of Bonneville is taking on new urgency. Climate models predict that the American West will become drier as global temperatures rise, but no one knows how much drier -- whether droughts will be merely a minor inconvenience, or catastrophes that could depopulate the likes of Salt Lake City, Las Vegas or Phoenix. "Predicting what's going to happen with rainfall is very tricky business," says Wallace Broecker, a prominent climatologist at Lamont-Doherty Earth Observatory in New York. "We still don't do it very well."

This is where Bonneville comes in. It provides a window into the past -- and possibly the future. As the last ice age wound down 30,000 to 10,000 years ago, temperatures seesawed wildly. Lake Bonneville is a perfect place to study how wetness in the Great Basin changed as a result of those temperature swings: The lake's water levels rose and fell by hundreds of feet during this time. Quade and his fellow geologists have come to the Silver Island Range to read this record of shifting moisture. Their initial results are not reassuring: As the West warms up, it will likely become substantially drier than it is today.