Image copyright Getty Images Image caption Some people believe Area 51 is home to crashed alien spaceships

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?

Image copyright Getty Images Image caption The Area 51 military base is restricted, meaning limited information is available on what goes on there

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.

He recently appeared on comedian Joe Rogan's podcast. There's also a Netflix documentary about him, which could have prompted renewed interest in his story.

Lazar said he had worked on taking apart a UFO and had read government documents describing alien involvement in life on earth.

Image copyright Getty Images Image caption Bob Lazar (centre) at the launch of his Netflix documentary Bob Lazar: Area 51 and Flying Saucers

He has no hard evidence or proof for what he's claiming, and no records of his alleged university physics degrees exist - but he definitely helped fuel the stories about Area 51.

As Thomas Bullard, author of The Myth and Mystery of UFOs says: "After trust between government and public eroded with Vietnam and Watergate, Roswell and Area 51 entered the popular vocabulary as bywords for official double-dealing."

Roswell is an area in New Mexico where an airborne object crashed in 1947.

The military says it was a weather balloon, but a theory has been widely held that it was actually a UFO that had crash-landed and then been covered up.

Despite the US government revealing in the 1990s that it was actually a nuclear test surveillance balloon, the incident still attracts lots of attention and conspiracy theories.

Area 51's influence on popular culture

Image copyright Alamy Image caption Paul the alien's look is a tribute to the classic Roswell alien

Paul - Simon Pegg and Nick Frost wrote and starred in this film, which sees two comic book enthusiasts travel across the US visiting sites where people have spotted UFOs. Along the way they meet an alien called Paul, who's been held prisoner in Area 51

- Simon Pegg and Nick Frost wrote and starred in this film, which sees two comic book enthusiasts travel across the US visiting sites where people have spotted UFOs. Along the way they meet an alien called Paul, who's been held prisoner in Area 51 Independence Day - The 1996 film focuses on a group of people who end up together in the Nevada desert after a worldwide attack by an unknown alien race. They take a captured alien to Area 51 and find out the government has been involved in a UFO conspiracy since the Roswell incident in the 40s

- The 1996 film focuses on a group of people who end up together in the Nevada desert after a worldwide attack by an unknown alien race. They take a captured alien to Area 51 and find out the government has been involved in a UFO conspiracy since the Roswell incident in the 40s The X-Files - During the sixth series of the sci-fi TV show, one of the storylines saw FBI special agents Fox Mulder and Dana Scully visit Area 51. They witness the flight of a mysterious looking craft that could be a UFO

Area 51 - A video game developed by Midway Games in 2005. It's a first-person shooter where you attack alien soldiers and a mutant alien that's been developed at the Air Base

Where and what is Area 51?

Image copyright Getty Images Image caption Area 51 is located in the Nevada desert

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.

What's it like at Area 51?

Image copyright Sinead Garvan Image caption Sinead's friend Jennifer Guertin had to get a picture next to this sign

"We obviously didn't make it on to the airbase but we tried our luck, driving as far into the desert as possible - only to see some quite scary signs from the US military warning us to stay well clear," says Sinead Garvan.

She was formerly our entertainment reporter at Radio 1 Newsbeat - but over the course of this story has become our chief alien correspondent. She went to Area 51 in 2014 as part of a road trip.

"All the areas surrounding the top secret military base are alien-themed.

"You're greeted with the sign 'Welcome Earthlings' along the extra-terrestrial highway.

"The petrol station is alien-themed with all the alien merchandise you could ever want - and a slightly bored-of-tourists lady behind the counter.

"Cartoon aliens and UFOs are all over the walls of the few buildings that are around, one of which is the famous Little Ale'inn motel.

"There are so many photo opportunities, but we did get a little shiver wondering if we were being watched all the way from the secure gates of Area 51."

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