Inside the enormous (4687 square miles) Nellis Bombing and Gunnery Range there is a 60 square mile block of land, sharing a border with the Nevada Test Site, at the centre of which is a large base. Its existence was not acknowledged by the government until 1995, in order to exempt it from Federal, State, and local environmental laws.
The site was set up in the 1950s for testing of the U-2 spy plane, due to its remoteness, existing facilities and the presence of a large dry lake bed for landings (the Groom Lake of the name). It is the traditional testing ground for experimental aircraft (skunk works) before they are publicly acknowledged, apparently including the SR-71 Blackbird, F-117 Nighthawk, and the hypothetical Aurora Spy Plane.
The facility and surrounding areas are also associated with UFO and conspiracy stories. In particular, some conspiracy theorists claim the storage and test flying of aliens spacecraft occurred there. The government has acquired nearby vantage points and added them to Area 51 in order to prevent snooping.
See also Area 19[?].