It started as a bit of a joke, but now the US Air Force has told people not to go near Area 51.
More than a million people have RSVP'd to an event on Facebook, threatening to storm the top-secret base in Nevada, which some believe is home to aliens.
Thousands have commented on the page, which reads: "We can move faster than their bullets. Let's see them aliens."
A spokeswoman for the Air Force has told The Washington Post it is "ready to protect America and its assets".
Facebook user Jackson Barnes wrote on the event page: "Hello US government, this is a joke, and I do not actually intend to go ahead with this plan".
"I just thought it would be funny and get me some thumbsy uppies on the Internet. I'm not responsible if people decide to actually storm area 51."
But the Air Force isn't seeing the funny side.
"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 spokeswoman said.
What are the theories about Area 51?
Some conspiracy theorists think the US government has information on alien life and UFOs that they are withholding from the general population.
They think the Area 51 base - which was formally recognised in 2013 - holds captured aliens, their technology and crashed UFOs (unidentified flying objects) - something denied by the government.
Theories about the base started in 1989 when a man named Bob Lazar claimed in an interview on US TV that he was a physicist who had worked in Area 51. Lazar said he had worked on taking apart a UFO and had read government documents describing alien involvement in life on earth.
Area 51 is part of a military base about 80 miles northwest of Las Vegas, Nevada.
The US government calls it the Nevada Test and Training Range, which is part of the wider Edwards Air Force Base.
Its primary use is hidden from the public, like many military bases across the US.
Storming the site as per the comments on Facebook might prove a bit difficult - it's restricted to the public and has armed guards patrolling the perimeters.
It's also impossible to enter the airspace above without permission from air traffic control, which won't be given.