The US 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.
The prank event on Facebook has attracted more than 1.2 million interested people after suggesting a mass of people attempt to run into the mysterious site at 3am on September 20.
The site is part of the vast Nevada Test and Training Range and has become the centre of UFO conspiracy theories.
The Facebook call says "They can't stop all of us" and "Let's see them aliens".
Nellis Air Force Base said in a statement that officials are aware of the Facebook posting and "any attempt to illegally access the area is highly discouraged".
The air force said it does not discuss its security measures and adds that the range provides "flexible, realistic and multidimensional battlespace" for testing and "advanced training in support of US national interests".
After decades of government officials refusing to acknowledge Area 51, the CIA released declassified documents in 2013 referring to the 20,000-square kilometre 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.
Secrecy surrounding the site has fuelled conspiracy theories among UFO enthusiasts and sprouted a small, alien-themed tourist industry in surrounding desert communities, including cafes, a motel and even a brothel.
Australian Associated Press