Area 51, also known as Groom Lake, is a secret military facility about 90 miles north of Las Vegas. The number refers to a 6-by-10-mile block of land, at the center of which is a large air base the government will not discuss. The site was selected in the mid-1950s for testing of the U-2 spyplane, due to its remoteness, proximity to existing facilities and presence of a dry lake bed for landings. Groom Lake is America's traditional testing ground for 'black budget' aircraft before they are publicly acknowledged. The facility and surrounding areas are also associated -- with varying levels of credibility -- with UFO and conspiracy stories.