Alien conspiracy theorists are convinced there is more than meets the eye at Area 51.
A video showing the strange noises over the Nevada US Air Force base posted on YouTube by channel UFO Seekers has racked up more than 40,000 hits already.
In the clip, F-22 fighter jets can be seen flying over the infamous airspace.
The stealth planes leave strange trails as they cut through the Nevada skies at speeds of up to 1,500mph, Dailystar.co.uk reports.
Then, as one accelerates away, a frighteningly loud double booming sound can be heard.
These fighter jets are a distraction from the real testing within the site.
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.
The new footage has sent alien conspiracy theorists into overdrive.
One viewer suggested that these fighter jets are a distraction from the real testing within the site.
And they even theorised the boom may not have come from the planes.
One YouTube poster said: “I think the regular fighter jets, F22’s etc, were there as a distraction, to cover for the real testing of experimental hypersonic aircraft.
“The burst of flame you captured is possible evidence of some type of Scram/Ram Jet type technology, maybe the cause of the double sonic boom?”
Another added to the conspiracy, saying: “Still waiting for the day for NASA and the whole world to officially tell us that extraterrestrial aliens have been living with us this whole time and that they are real.”
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.