"We may not be alone." That is the belief of former Pentagon official Luis Elizondo who has cited "compelling evidence" of extraterrestrial life.

His comments to CNN this week came a few days after the US Defence Department admitted, for the first time, to funding research into the paranormal with the $22m (£16.4m) Advanced Aerospace Threat Identification Programme, established in 2007.

Harry Reid, former Democrat senator in Nevada, said the programme, which analysed reports of UFO sightings and collected audio and video evidence of mysterious aircraft, had gathered "plenty of evidence to support asking the questions" although no answers have been found.

Conspiracy theories about aliens and UFOs have been rife, both on and off the net. Here are four of the biggest from the past century.

Roswell 'weather balloon'

In July 1947, an unidentified flying object crash-landed north-west of Roswell, New Mexico. Debris was scattered across an area three-quarters of a mile long and a few hundred feet wide, and included small bits of metal and weightless "I"-beam-like structures that would neither bend nor break.

Rowell Army Air Field released a press release announcing a "a flying disk" had been found. But the very next day a second release was issued, saying the 509th Bomb Group had mistakenly identified a weather balloon as flying saucer wreckage.

The front page of the Roswell Daily Record on July 9, 1947. Wikimedia Commons/drew peacock

An AP story run by numerous newspapers after the official line was changed read: "Reports of flying saucers whizzing through the sky fell off sharply today as the Army and the Navy began a concentrated campaign to stop the rumors."

It also said the headquarters bosses in Washington "delivered a blistering rebuke to officers at Roswell".

Things went quiet until the 1970s, when conspiracy theorists began to claim that in fact one or more alien spacecraft had landed and that the military had recovered the extraterrestrial occupants and covered up the discovery. The now infamous classified US Air Force facility Area 51 (see below) is often believed to have been where the debris was stored and examined, along with other alien spacecraft.

In the 1990s, reports published by the US military said that the object was a nuclear test surveillance balloon from Project Mogul (an operation to detect Soviet atomic tests) but the conspiracy theories persisted.

Britain's Roswell - Rendlesham Forest

Two US airmen on a security patrol reported seeing strange lights on 26 December 1980 near their base at RAF Bentwaters in Suffolk. A memo by Col Charles Halt reported a metallic-looking, glowing object with coloured lights. Strange lights were also seen on other days that month.

In 2015, Halt came forward to say that former service people had provided him with written statements to support his story since their retirement. He told the BBC: "I have confirmation that [Bentwaters' radar operators] saw the object go across their 60-mile [96km] scope in two or three seconds, thousands of miles an hour. He came back across their scope again, stopped near the water tower, they watched it and observed it go into the forest where we were.

"At Wattisham, they picked up what they called a 'bogie' and lost it near Rendlesham Forest. Whatever was there was clearly under intelligent control."

But sceptics have said the lights could have been a series of less alien things, from a downed Soviet spy satellite to pranks, Orford Ness lighthouse or military testing.

5 of the most popular conspiracy theories debunked NASA, Reuters, Getty Images, Videoblocks, Giphy

The Ministry of Defence said the matter was not a security threat and declined to investigate further. When a substantial file of information on the incident was discovered, the MoD was accused of a coverup but it later turned out the folder was mostly full of responses to inquiries from the public and internal correspondence.

A three-mile woodland trail now marks the site, created by the Forestry Commission because of high levels of interest in the area. The commission said: "Many people think these mysterious events are the most significant UFO incident to have occurred in the UK [but there is] no tangible evidence on the ground."

UFO 'saved earth' from devastating meterorite over Russia

Asteroid explosion near the Russian city of Chelyabinsk in February 2013. Alan Fitzsimmons

After a 20-metre meteorite exploded over Chelyabinsk, Russia, in 2013 some conspiracy theorists claimed a UFO shot it down and saved the Earth. Early speculation claimed a Russian missile had shot the meteorite down but the authorities never confirmed this.

Alexander Komanev, coordinator for the Russian UFO community in Yekaterinburg, said: "At first, we also believed that the Chelyabinsk meteorite was just an ordinary meteorite, a cosmic body. But facts began to emerge. On the internet began to appear videos, at least three of which were similar, on which you can see how an object catches the meteorite.

"Such a number of videos, made from different angles makes us believe that something blew up the meteorite."

Area 51

Area 51: CIA Only Admits Existence.
Area 51, UFO Sightings: CIA Only Admits Area 51 Existence, Not UFOs or Aliens. Tumblr/minus229k1

Area 51 has been called one of the worst-kept secrets on earth. The US Air Force facility in Nevada is a highly classified remote detachment of Edwards Air Force Base which was not even acknowledged by the US government for a long time, stoking the flames of curiosity and conspiracy.

Its existence was finally confirmed by the CIA in 2013 when declassified documents revealed that due to its remote location in the desert, it was used as a testing area by the CIA, the US Air Force and defence contractor Lockheed.

Regardless, people still claim that Area 51 is used, as they say it was in the Roswell incident, for reverse-engineering spacecraft, cloning extraterrestrials, and even for filming the "fake" Moon landing.

Multiple witnesses have claimed to have had extraterrestrial experiences at or near Area 51. A double sonic boom heard over the base earlier this month led to people claiming it was proof the CIA was conducting secret tests of alien or hypersonic aircraft technology.

Earlier this year, the "deathbed confession" of a reported former CIA agent was released in which he said: "There were alien craft, including the Roswell UFO and live aliens."