It all started as a joke – but now authorities in Nevada are wondering how many people will show up in the desert later this month to storm Area 51.

Millions of people responded to a Facebook post saying, “Storm Area 51, They Can’t Stop All of Us” -- in search of aliens. All claim they are going there on Sept. 20.

The man who made the Facebook post is now trying to make it a music festival.

"I'm trying to advocate against the storming as much as I can,” says Matty Roberts. “I just want a gathering of all these weirdoes in the desert."

Roberts says in his best estimate; 20,000 people will actually show up.

An Air Force spokesperson tells CNN any attempt to illegally access military installations or military training areas is dangerous.

The Air Force claims the 60 secretive square miles are used for 'earthling' aircraft testing, but one recent survey found 54 percent of Americans believe the government knows more than they're telling us about aliens.

The nearest civilization is Rachel, Nevada – which has a population of 52.