From a mountainside overlooking Utah's Great Salt Lake, a time traveler going back 20,000 years would witness a dramatically different landscape. The shallow, salty lake would have expanded into a massive freshwater lake, reaching nearly 1,000 feet (305 meters) deep and covering more than 10 times its current area. Mountain ranges would have become islands, and the shoreline would have stretched far beyond its modern limits. Traveling back an additional 120,000 years would reveal another giant lake. A new study led by researchers at the USC Dornsife College of Letters, Arts and Sciences, published in Paleoceanography and Paleoclimatology, traces these ancient transformations through sediments buried beneath the lake. By analyzing nearly 240,000 years of sediment layers, scientists found that these enormous lakes were brief periods of change from the lake's usual extremely salty state. Both giant lakes appear to have followed a similar pattern: as the climate warmed and dried, the water receded, salinity increased, and salt deposits formed on the lakebed. However, the timing of the older transition is less precise. Rachel So, a recent Ph.D. graduate from USC Dornsife and a corresponding author of the study, compares the record to watching a puddle that remains nearly the same size for most of an hour, briefly swells into a pond, and then shrinks again. "If you scaled this up to the size of the present Great Salt Lake, that's probably what it looked like," she says. "For most of the past 240,000 years, the lake stayed about the same size but briefly grew 10 times larger on two separate occasions." Ancient sediments reveal a lake transformed. Scientists have long known that Lake Bonneville once covered much of western Utah during the last ice age, and its ancient shorelines remain visible across the landscape. However, these shorelines provide only snapshots, not a continuous account of changes in the lake's size and salinity over time. To understand the continuous record, researchers turned to a nearly 400-foot (122-meter) sediment core drilled from the lakebed in 2000. Its layers preserve a record stretching back about 236,000 years. By measuring radioactive decay in minerals, the team dated the sediment layers, using a kind of geological clock. They also analyzed molecules from microorganisms that once lived in the lake, which change in abundance with salinity, helping them gauge whether the water was fresh, brackish, or extremely salty. The sediment record shows two major interruptions in the lake's long hypersaline history. From roughly 30,000 to 16,000 years ago, the lake expanded into Lake Bonneville. Earlier, around 140,000 to 135,000 years ago, an even deeper lake called Little Valley existed. Shoreline evidence suggests both lakes approached 1,000 feet (305 meters) in depth, though Little Valley may have been somewhat brackish and lasted less than half as long as Bonneville. "The fact that the core spans two and a half glacial cycles allows us to see two fresh-to-salty transitions," said senior author Sarah Feakins, a professor of Earth sciences at USC Dornsife. "It allows us to compare the two fresh lake events for their duration and freshness, to see how fragile and fleeting these moments are." Similar wet-to-dry shifts are seen in records from Nevada, California, and Arizona, suggesting that Great Salt Lake responded to regional climate changes. Modern Great Salt Lake faces pressures its ancient predecessors did not. Human water use, mainly for agriculture, reduces the amount of water reaching the lake, while human-caused climate change increases evaporation. Feakins notes that cooler temperatures in the past slowed evaporation and more frequent storms brought additional water into the basin. Lake Bonneville filled "one storm at a time," but as temperatures rose, evaporation increased and storms became less frequent, causing the lakes to shrink and their water to become saltier until minerals formed crystals on the lakebed. Great Salt Lake has no outlet, so water leaves mainly through evaporation, making it highly sensitive to changes in temperature, precipitation, and river flow. Feakins warns against using ancient climate changes to justify today's lake shrinkage. "People shouldn't use the defense that 'climate change happened naturally in the past and so the lake shrank' to justify shrinking lakes today as normal or a natural phenomenon," So added. Despite challenges in dating older layers and reconstructing past temperatures due to the lake's high salinity, the sediment core provides a nearly continuous record of how the lake responded to major shifts in the region's water balance, showing how a vast freshwater lake can give way to salt when the climate warms and dries.