North end of Yucca Flat at the Nevada National Security Site. Most underground nuclear tests were conducted in this valley. Underground testing often leaves visible evidence on the surface in the form of subsidence craters in varying dimensions. Subsidence craters are depressions on the surface that occur when the roof of the blast cavity collapses into the void left by the explosion. The size of the subsidence crater depends on the yield of the device, the depth of emplacement, and the geological characteristics of the surrounding soil. Yucca Flat is a closed desert drainage basin, one of four major nuclear test regions within the Nevada Test Site (NTS), and is divided into nine test sections: Areas 1 through 4 and 6 through 10. Yucca Flat is located at the eastern edge of NTS, about ten miles north of Frenchman Flat, and 65 miles from Las Vegas, Nevada. Yucca Flat was the site for 739 nuclear tests and has been called "the most irradiated, nuclear-blasted spot on the face of the earth".