Area 51 the base extends further east. After leaving the restricted area, Groom Lake Road descends eastward to the floor of the Tikaboo Valley, passing the dirt-road entrances to several small ranches, before converging with State Route 375, the "Extraterrestrial Highway", south of Rachel. Area 51 shares a border with the Yucca Flat region of the Nevada Test Site, the location of 739 of the 928 nuclear tests conducted by the United States Department of Energy at NTS. The Yucca Mountain nuclear waste repository is southwest of Groom Lake. Groom Lake is a salt flat in Nevada used for runways of the
Area 51 aerospace magazine "The Space Review" titled "Astronauts and Area 51: the Skylab Incident". The article was based on a memo written in 1974 to CIA director William Colby by an unknown CIA official. The memo reported that astronauts on board Skylab 4 had, as part of a larger program, inadvertently photographed a location of which the memo said: Although the name of the location was obscured, the context led Day to believe that the subject was Groom Lake. As Day noted: The memo details debate between federal agencies regarding whether the images should be classified, with Department of Defense agencies