The U.S. Air Force has warned people against participating in an internet joke suggesting a large crowd of people �storm Area 51,� the top-secret Cold War test site in the Nevada desert.

A prank event on Facebook that�s attracted more than 1 million interested people suggests that a mass of people attempt to run into the mysterious site at 3 a.m. on Sept. 20.

The site is part of the vast Nevada Test and Training Range and has become the center of UFO conspiracy theories.

The Facebook event jokes �they can�t stop all of us� and �Let�s see them aliens.�

Nellis Air Force Base said in a statement that the Air Force is aware of the Facebook posting and says �any attempt to illegally access the area is highly discouraged.�

The Air Force says it does not discuss its security measures and that the test and training range provides �flexible, realistic and multidimensional battlespace� for testing and �advanced training in support of U.S. national interests.�

After decades of government officials refusing to acknowledge Area 51, the CIA released declassified documents in 2013 referring to the 8,000-square mile (20,700-sq. kilometer) installation by name and locating it on a map near the dry Groom Lake bed.

The base has been a testing ground for a host of top-secret aircraft, including the U-2 in the 1950s and later the B-2 stealth bomber.

But secrecy surrounding the site has fueled conspiracy theories among UFO enthusiasts and sprouted a small, alien-themed tourist industry in surrounding desert communities, including alien-themed cafes, an alien-themed motel and an alien-themed brothel.

(AP)