In 1865, Mormon settlers built a farming town at the confluence of two rivers in the Nevada desert. They called it St. Thomas. For seventy-three years, people lived there — grew cotton, figs, and melons, raised families, opened an ice cream parlor on the road between Salt Lake City and Los Angeles. Then the federal government built Hoover Dam, and Lake Mead swallowed the town whole. The last resident walked out in 1938 with the water at his doorstep.
For sixty years, St. Thomas sat at the bottom of the reservoir. Then the lake began to shrink.
Today, the foundations of the drowned town rise out of cracked mud in the Nevada desert — a ghost town that was underwater for most of the twentieth century, now exposed by the worst drought the Colorado River basin has seen in over a thousand years. The ruins sit inside Lake Mead National Recreation Area, where anyone can walk the old streets and stand on the concrete slab where the schoolhouse used to be. The same water crisis threatening Las Vegas and the entire American Southwest is slowly giving this abandoned town back to the desert.
This is the full story. Who built it, why they left, what the government paid them, and what the receding lake has revealed.

Sources:

Marc Reisner, Cadillac Desert: The American West and Its Disappearing Water (Penguin, revised ed. 1993)
Bureau of Reclamation, The Story of Hoover Dam (U.S. Department of the Interior)
National Park Service, "St. Thomas Historic Site," Lake Mead National Recreation Area
David Owen, Where the Water Goes: Life and Death Along the Colorado River (Riverhead Books, 2017)
Clark County Museum oral history archives, Henderson, Nevada
U.S. Geological Survey, "Status of Lake Mead Water Levels," Colorado River Basin monitoring reports