Those mineral bathtub rings are an unexpected gift to anyone who wants to understand the history of Bonneville, and of wetness in the West. But they aren't foolproof. Now and then, the lake's water became fresh enough that it stopped laying down minerals. McGee and Quade see it in the rocks that they've already dated: One layer is sometimes far younger than the layer just beneath it, with no sign of the 1,000 years in between. If you want a record of those thousand silent years, you have to look for other indicators, such as the shells of critters that lived during that time, or layers of mud that settled into caves.

One day, McGee and Quade go looking for these other clues an hour's drive east of the Silver Island Range, on a 2,700-square mile forbidden zone called the Utah Test and Training Range, run by the U.S. Air Force.

Our military-owned Chevy Suburban bumps down dirt tracks that wind through a desert plain lush with waist-high grass and sagebrush, past a 1970s-era F-4 Phantom fighter-bomber, a row of Army Jeeps perched on concrete platforms, and a concrete building shattered on one end. Some of this is old hardware of no further use to the military; other objects are high-quality replicas built by a special military team using plywood and other cheap materials to create realistic stage sets for soldiers as they practice firing machine guns or dropping bombs.

Cathedral Cave, today's destination, sits several hundred feet above the plain, sunk into the base of a limestone cliff. It takes 10 minutes of breathless scrambling up a steep slope to reach it. A band of fossils runs across the cliff and intersects the cave -- Mesozoic coral broken off in an ancient storm, strewn on the seafloor and frozen for eternity in three inches of petrified mud. Cathedral Cave sat as far as 750 feet below the surface of Lake Bonneville. The story of this year's expedition begins deep in its bowels.

Quade sidefoots his way down a dusty slope into the cave. He walks through a vaulted room whose walls are covered in knobby tufa reminiscent of organ pipes in a church, and picks his way to the dark, narrow rear of the cave. The light of his headlamp falls on a crystalline crust of stone that covers the walls and ceiling. The beige crust has broken in places, revealing a six-inch cross-section.

These beige crystals formed all over the cave, but only here in its darkest recesses are they free of the silt or fossil algae that would complicate Quade and McGee's chemical analyses. Quade accidentally stumbled upon these super-clean crystals when he first visited Cathedral Cave in 1994. "I wandered back here and was mesmerized," he says. The rocks that sat for 13 years in his sample chest came from this spot.

Alternating layers of calcite and aragonite record 9,000 years of history when the cave was flooded. These mineral layers are capped with evenly spaced calcite knobs the size of coat buttons. On top of them lies a fine dusting of white crystal aragonite, like an autumn morning frost. The buttons formed over a period of 600 years as the lake contracted, grew salty, and fell below the cave; the frosting reveals one final hurrah when the lake briefly crested again above the cave for another 200 years before drying out for good. McGee and Quade are analyzing them for any clues that they might hold. But for the moment they are simply beautiful. "It's good that this cave is protected," says Quade. "Otherwise people would have destroyed this stuff."

The geologists already have samples of these minerals; they've come today to tease apart layers of silt, animal remains, and debris on the cave floor. The group begins digging two pits. Dust that tastes of acrid rat urine billows into the air.

Taking turns, they dig through two feet of dirt; through rocks fallen from the ceiling; more dirt; several inches of hard tufa which are chipped away with rock hammers; and below that, a layer of mud. A debate ensues about whether the mud was laid down when the lake was only a few feet above the cave, or several hundred feet above it.

"You want to taste some?" Quade asks. He hands over a pinch of mud that's creamy to the touch. But the tongue's exquisite sensation, a sort of oral Braille, magnifies its grit, revealing sharp-edged grains of silt too fine for calloused fingers to sense. These grains must have washed in during rainstorms from higher up on the mountain, when the cave was just below the shoreline - or so the geologist's diagnostic palate would indicate.

Guleed Ali, a Ph.D. student from Lamont-Doherty, finds a millimeter-long snail shell in the damp silt. "Two shells is going to be enough for a carbon date," McGee tells him -- giving them a chance to date when the lake level sat just above the cave. Pretty soon everyone is sitting, sorting through bits of silt on notebooks and clipboards for other clues that might reveal its age.

Madsen holds up a mud clod containing tiny white shells of crustaceans called ostracods, which give clues about the salinity of the lake. "The way you tell the different species apart is the size of their penis," he says. "Those aren't preserved here, so we don't have to worry about it." A few moments later, he cups the minuscule vertebra of a fish in his hand -- "probably a Utah chub or a redside shiner," he says.

Cathedral and other caves have provided a record of how the region's ecosystems evolved as Bonneville climbed up and down the mountains starting around 30,000 years ago. Archaeologists have found thick mats of limber pine and spruce needles laid down by water. They have also found seeds and stems of meadow- and marsh-dwelling currant bushes, cinquefoil and bulrush crammed into packrat burrows and cemented together with crystallized urine.

The caves also reveal when humans arrived, leaving behind charcoal, antelope bones, scraping stones, and in the case of Danger Cave in the Silver Island Range, a veritable biblioteca of dried turds. "Tens of thousands of turds of all shapes and sizes," said Madsen when we visited Danger Cave several days earlier. Archaeologists used to reconstitute them in water to study them: One team of researchers tried to distinguish their culinary contents based on aroma. Each one represents a day in the fecal diary of the people who arrived as Lake Bonneville waned around 14,000 years ago. There are short, squat turds stuffed with bat hair that seem to cry out for Metamucil, and high-fiber, narrow-gauge turds made almost entirely of a marsh plant called pickleweed that grew near the water's edge.

The pronghorn antelope that the early human inhabitants ate still roam the area today. But many of the other species preserved in Danger and Cathedral Cave are nowhere to be seen. The limber pine and spruce climbed several thousand feet up the slopes of the Silver Island Range as they chased the retreating rainfall. Eventually, those islands of cool, wet climate evaporated off the tops of the range's 7,000-foot peaks, and the trees became extinct around here. Today, only junipers and one tiny stand of piñon pine grow atop the Silver Island Range. The only limber pine and spruce in the region reside in the Pilot Range, the Oquirrh Range, the Wasatch Range and other mountains where peaks above 9,000 feet still wring enough moisture from the air to sustain the trees through hot summers.

The Great Basin in the time of Bonneville wasn't just a wetter place. The greater amount of water flowing and seeping through its mountains and hills supported what biologists call a more productive ecosystem: It could sustain more tons of plants and animals per acre than today. That means it could probably have sustained more humans, too.