Area 51 is a secret US government facility in the Nevada Desert. It is an Air Force base, and covers most of the area due south of Dry Groom Lake. Given the secretive nature of the area, popular belief is that the US government is conducting experiments there on captured extraterrestrial technology and, possibly, live aliens.
It's the main testing facility for new aircraft designs.
However, in the far more interesting world of Perfect Dark, there really are aliens at Area 51. Captured, imprisoned and awaiting dissection. Only you can save the Maians prisoner from a grisly fate!
This section of the extensive facility known as Area 51 is based in the foothills around Groom Dry Lake. Exterior helipads and communication towers hint at the size of the complex beneath ground level.