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…