Black Canyon of the Colorado is the gorge on the Colorado River where Hoover Dam was built along the Nevada-Arizona state line. Formed about 15 million years ago during the Miocene Basin and Range uplift, it takes its name from the dark volcanic rock that gives the canyon its severe appearance. Its notable features include Sauna Cave, a miners’ tunnel from dam construction lined with calcium carbonate crystals and heated by a hot spring, as well as geothermal waters in Boy Scout Canyon and hot springs on the Arizona side. Today, the river through the canyon is part of the Black Canyon Water Trail, reflecting its geological and scenic significance.