Volcanic forces more than 10 million years ago created the Snake River Canyon, but the Bonneville Flood, the second largest flood in world history, reshaped it into its present form. Lasting nearly eight weeks, the flood sent a four hundred-foot wall of water through the canyon at nearly 100 miles per hour and submerged the entire canyon as it raced toward the Pacific Ocean, carrying and crashing boulders as large as semi trucks. During the last Ice Age, Lake Bonneville covered 32,000 square miles, occupying one-third of Utah and exceeding the size of today’s Lake Michigan, until about 15,000 years ago when an alluvial gravel dam at Red Rock Pass near Pocatello, Idaho, suddenly failed and rapidly drained the lake into southern Idaho. The flood rounded river gravel as it swept rocks hundreds of miles, then deposited them across the land where the water slowed. Farmers in southern Idaho near the Snake River later found these rounded stones, from baseball-sized rocks to half-buried boulders weighing hundreds of tons, and jokingly called them Idaho “petrified watermelons.” Bonneville Flood gravel remains scattered across southern Idaho to the Oregon border, while the Great Salt Lake and the Bonneville Salt Flats mark the former site of this vast glacial lake.