In December 2017, UFO seekers sparked claims the CIA was hiding alien life in Area 51 after a double sonic boom was caught on camera.
The heavily guarded Area 51, deep in the Nevada desert, is at the centre of many alien-related conspiracy theories.
It is claimed that alien flying saucers crashed on Earth, such as the alleged 1947 Roswell incident, and that the wreckage and even corpses were taken to Area 51 for experimentation and to be analysed away from public view.
One theory suggests that USAF scientists have tried to emulate the alien technology to create their own top-secret spy craft capable of travelling into space at the base.
Area 51 has been used by American aviation authorities since 1955 and is closed off to those without top-level security clearance.
However, the US Government did not even admit the base existed until 2013, in a series of documents released as part of a Freedom of Information request, and it is heavily protected by armed guards.
Thousands visit the remote area of Nevada every year in the hope of seeing ETs.
Many more sceptical people believe Area 51 is so top-secret, because it is where the USAF designed and tests experimental new spy planes and air craft.
It is accepted that the USAF does secretly test new aircraft inside Area 51, with the U-2 spy plane, SR-71 Blackbird, and the F-117A Nighthawk stealth fighter all developed there.
Nigel Watson, author of the UFO Investigations Manual, said: “Area 51 has always been a magnet for those who believe the US Government knows a lot more about UFOs than they are prepared to reveal to the public.”
The area is so mysterious that US presidential hopeful Hillary Clinton made a shock promise that she would declassify all secret Area 51 files.