Pupfish is any of about 30 species of small fish that live mainly in springs and streams in the southwestern United States and Mexico. Pupfish have inhabited these waters for at least 50,000 years, before the end of the most recent ice age. At that time, rivers and lakes covered the area, much of which is now desert. Isolated groups of pupfish survived after most of the water had dried up. One species, the Devils Hole pupfish, has lived over 20,000 years in a deep spring in Nye County, Nevada. This area is now in Death Valley National Park.