Area 51 is located about 90 miles north of Las Vegas. It is a highly secretive American air force base. The exact use of Area 51 is unknown as it is strictly guarded and cut off from the public.
In fact, it was only publicly acknowledged by the CIA in 2013, when the CIA released the official history of some of its projects in response to a Freedom of Information Act request.
60 declassified documents were handed over, which particularly focused on activity in the 1960's and 1970's and the need to develop stealth (spy) aircraft to monitor the Soviet Union. Other documents described the need to take photographs of Area 51 by satellite to see what the Russians may also see if they were spying on them with satellite.
One thing these documents make no mention of is UFOs. That's right, no mention of flying saucers, little green men or of anything remotely to do with extraterrestrials. So why is Area 51 so commonly linked to aliens?
Well there's a couple of reasons:
The Roswell Cover-Up
In July 1947 there was a crash in Roswell, New Mexico. Many believe it to be that of a UFO, specifically a spacecraft and some even claimed to see aliens leave the craft. However, the police and US government claimed it was actually a crashed military air balloon, and absolutely nothing to do with extraterrestrials.