The couple, who posted the video to YouTube, were stunned by the huge plume of dark smoke that appeared to form a mushroom cloud when viewed from afar. The pair, called Jimmy and Jackie, drove through the Nevada desert to the base that the US government only confirmed existed in 2013, despite being one of the country's worst kept secrets, and spotted the smoke while approaching from Groom Road. Jimmy said in the video: "We are still driving down groom road here and we noticed that there is like weird smoke over here. What could be going on?" When they reached the main gate, which has warning signs not to cross, there was another group of four people watching the smoke who were being monitored by a white security jeep parked up on a hill. Some conspiracy theorists claim the authorities conceal evidence of aliens visiting Earth from the public within the base. It has been claimed a "flying saucer complete with aliens" said to have crashed near Roswell, New Mexico, in 1947 was taken and stored there for examination. Some suggest they try to reverse engineer alien space craft there, but more grounded researchers claim it is just a testing site for new aircraft technologies. British UFO expert Mike Oram even claims he was abducted by aliens in league with military personal during a visit to Area 51.

FINDINGUFO•YouTube The huge mushroom cloud and (inset) military keep out signs

FINDINGUFO•YouTube First look at the smoke on Groom Road

The base is so secretive that Democrat presidential frontrunner Hillary Clinton has vowed to find out what goes on there if she makes it into the White House. Other conspiracy theorists claim a controversial High Frequency Active Auroral Research Program (HAARP) may have been restarted at Area 51. HAARP was a weather ion research programme jointly funded by the US Air Force, US Navy, the Defense Advanced Research Projects Agency (DARPA) and universities. It began in 1993 and at the height of conspiracies, some theorists claimed it could change weather patterns, cause earthquakes, droughts and storms. Actually, based on a sub-arctic facility, an Air Force-owned site near Gakona, Alaska, HAARP's purpose was to analyse the ionosphere and investigate the potential for developing ionospheric enhancement technology for radio communications and surveillance. It temporarily ceased in May 2013 and was officially closed a year later. As the couple drove away from the base they were caught by a sudden downpour of rain. Jimmy said: "We've got a big cloud coming over us. It is probably radioactive rain from whatever they are burning over there."

FINDINGUFO•YouTube A group of people were already staring at the cloud

We are still driving down groom road here and we noticed that there is like weird smoke over here. What could be going on? Area 51 investigator Jimmy

Their trip took place in 2012, but the footage is making waves after being posted on paranormal YouTube channel FindingUFO. Online posters speculated over what could have caused the fire. One appeared to believe it could be connected to Haarp. Translated from French, they posted: "Make Rain! Carbon dioxide and silver iodide. Geo-engineering Desert! "The aliens are a diversion. A type of scientific research?" But Richard Mather posted: "Looks like there burning their toxic waste again. Well the president signed an executive order that they can do what they like." Jimmy said he asked at a cafe in the nearest town Rachel, and staff said it was probably a "brush fire." There are reports that on April 4 1999 there was a huge fire in the mountains by Area 51, that was officially said to have been caused by a lightening strike.

FINDINGUFO•YouTube Area 51 security jeep ringed in front of smoke and (inset) close up

Top 10 Conspiracy Theories Wed, March 8, 2017 From mind control, to the missing Malaysian airways flight MH370 Play slideshow AFP/Getty Images 1 of 10 In 2011, Osama Bin Laden was killed by US Navy Seals and buried at sea, but now conspiracy theorists claim that it was all a lie and he is still alive