Overview:It's a mysterious complex of buildings and runways that has been photographed by U.S. and Russian satellites and documented on the cover of Popular Science magazine. Yet the U.S. Government says the Area 51 base does not even exist. From a secret gate at the Las Vegas airport, workers are flown daily to and from the base in aircraft with blacked-out windows. Airplanes are warned they'll be shot down if they fly over Area 51. Signs authorizing the use of deadly force surround the base to keep intruders away. Unmarked helicopters and hummers are on constant patrol. What are they guarding? A scientist who once worked there says Area 51 houses confiscated UFOs. Now, a visitor to the base says he actually examined an alien UFO engine there. Is Area 51 truly a spaceport for alien craft?