IT is a highly classified United States Air Force base, shrouded in such intense secrecy that it has spawned a litany of conspiracy theories about what exactly goes on there. Now, security is higher than ever after millions of people signed up to a Facebook event to storm Area 51 today.

Area 51?

The official name for the top secret base, deep in the Nevada desert - around 100 miles north of Las Vegas - is Homey Airport.

For decades, the US government denied it even existed, but six years ago, CIA documents acknowledged the base and revealed its codename of Area 51.

Hasn’t it got something to do with UFOs?

It has a long-standing association with aliens and conspiracies, with links to the Roswell incident in New Mexico in 1947, when officials said a weather balloon crashed. But in the ensuing decades, ufologists began touting theories that alien spacecraft had actually crash-landed in Roswell and that aliens and the craft had been recovered by the military and taken to Area 51.

And that theory took flight?

It was aided by revelations by physicist Bob Lazar - the subject of the current Netflix documentary, “Bob Lazar, Area 51 & Flying Saucers” - that he had reverse engineered an alien ship there.

What do officials say the base is for?

It’s exact purpose is not fully known and the zone around it is heavily restricted, with any trespassers risking fines and prison sentences for breaching it.

It is thought, though, that Area 51 supports the development and testing of experimental aircraft and weapons systems.

People plan to storm it?

In June, a Facebook event, “Storm Area 51, They Can’t Stop All of Us”, saw 2.1 million Extra Terrestrial enthusiasts respond saying they were going and another 1.5 million respond saying they were interested in attending.

The invite read: “We will all meet up at the Area 51 Alien Center tourist attraction and coordinate our entry”, adding: “Let’s see them aliens.”

The original poster freaked out?

Matty Roberts, a 20-year-old Californian student, realised the event had escalated out of hand and posted online: "Hello US government, this is a joke, and I do not actually intend to go ahead with this plan…I'm not responsible if people decide to actually storm area 51.”

But FBI agents knocked his door to question him about his intentions.

So he has disassociated himself from the plan?

He has tried to and in a bid to divert people from the original 'event', he set up a festival "Alienstock", supposed to take place in a small local town called Rachel. Roberts now fears this could in itself become a "possible humanitarian disaster”, alongside the original event, with both placing a huge strain on local law enforcement in Lincoln County. To prepare, officials there have pre-signed an emergency declaration, fearing as many as 50,000 may ultimately descend on the area.

So security is at its highest level?

The Air Force have issued a warning to potential trespassers that “the US Air Force always stands ready to protect America and its assets”.