Over 2 million users have RSVP'd to an alien-hunting Facebook event encouraging to storm the most secretive military base in the United States where, per passionate conspiracy theorists, the government is housing ETs. Now, other truth-seeker claims there might be "an underground city" beneath the notorious Area 51.

Axar.az reports citing foreign media that Blake Cousins, a renowned conspiracy enthusiast from Hawaii running the YouTube channel "third phase of the moon", claims that former Area 51 employees confided in him that there was an "underground city" below the famed US military base, which is located in the Nevada desert.

"If one person or a group of people were able to get in and open the doors into the subterranean levels, you will see a huge underground city that has never been seen before on our planet. This is something so big that the ground-level Area 51 is just the tip of the iceberg. The subterranean level would have to be at least 50 storeys down, from what I hear, and they extend up to the Pacific Ocean and other parts of the United States", he told.

Area 51, a US Air Force base, was established in the 1950s to test secretive aircraft programmes, but the US government didn't even acknowledge the facility's existence until 2013 when the CIA declassified it following a Freedom of Information Act request.

The mystery surrounding the base has spawned bizarre theories as to what it may harbour, with multiple rumours suggesting the military was keeping classified data about extraterrestrials and UFOs.