More than half a million Facebook users have signed up to storm Area 51 to “see them aliens” at the U.S. Air Force base in southern Nevada that’s long been shrouded in mystery for those who believe it’s a holding site for otherworldly creatures.

It all may be a joke or a lark for many of the supposed UFO seekers. But federal officials aren’t laughing about the possibility of people actually showing up at the highly classified military base, which is a remote detachment of California’s Edwards Air Force Base.

An event page called “Storm Area 51, They Can’t Stop All of Us” had garnered nearly 670,000 recruits as of Saturday. Almost 620,000 others have indicated they’re “interested.”

The purported raid is set for Sept. 20 between 3 a.m. and 6 a.m., with the “details” section noting, “If we naruto run, we can move faster than their bullets.” The reference is to Naruto Uzumaki, a Japanese manga character who appeared to run at the speed of light while leaning forward and stretching his arms behind his back.

According to ComicBook.com, a news site covering comics, TV shows and movies, a social media user who goes by the name “SmyleeKun” and who devised the Facebook page is “best known for posting memes and streaming to fans on Twitch,” a live streaming platform used by gamers.

In other words, it’s almost certainly a prank. But it’s also caught the attention of the Defense Department.

Air Force spokeswoman Laura McAndrews, in comments to The Washington Post on Friday, warned anyone seriously intending to follow through with storming the site to think twice.

″[Area 51] is an open training range for the U.S. Air Force, and we would discourage anyone from trying to come into the area where we train American armed forces,” she said. “The U.S. Air Force always stands ready to protect America and its assets.”

Though the government had denied the existence of Area 51 for years, in 2013 the CIA acknowledged it as real. However, it has only been described as an aviation test site.