More than one million people who have signed up to raid Area 51 in the hope of seeing evidence of aliens have been warned that the US air force "stands ready to protect America and its assets".
Despite the creator of the Facebook event having confirmed it is a joke, it has attracted a huge number of devotees since it popped up earlier this month.
So many have committed themselves to storming the military base in Nevada on 20 September that the US air force has felt the need to issue an official warning, having previously only said it was aware of the post.
In a statement to The Washington Post, a spokeswoman said: "Area 51 is an open training range for the US air force, and we would discourage anyone from trying to come into the area where we train American armed forces.
"The US air force always stands ready to protect America and its assets."
Huge secrecy surrounds Area 51, which is not open to the public and is under 24-hour surveillance.
The base, located about 150 miles from Las Vegas, was first used to develop U2 spy planes in the 1950s.
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That programme finished after the U2 was put into service around 1956 and the base has since been used for testing other military aircraft, but conspiracy theorists say the site is also home to remains of crashed UFOs.
Peter Merlin, an aerospace historian who has written extensively about Area 51, said the facility is "strictly a place for testing and evaluating aircraft and associated weapons systems".
He told NBC News anyone who shows up on 20 September may find themselves more at risk from the unforgiving desert terrain rather than the bullets of those who guard the facility.
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