In one of the most extraordinary alien conspiracy theories to date, many so-called “truth seekers” believe there is a seven-storey underground complex beneath Mount Archuleta on the Colorado-New Mexico border near the town of Dulce.

Dulce, New Mexico, is a small town with a population of just under 3,000, and is the tribal headquarters of the Jicarilla Apache Reservation. 

The remarkable myth centres around claims that US military personnel and aliens cooperate together in the nearby base, with the government given alien technology in return for allowing extraterrestrial races to abduct and experiment on limited numbers of humans.

And it claims men hired to complete the base by drilling huge tunnels ended up in a violent stand off with “grey aliens” and 57 out of the 60 humans were slaughtered, in 1979.

The theory began in the mid to late 1970s when Gabe Valdez, a former New Mexico State Police trooper, claimed to find strange activity around mutilated cattle, including discarded gas masks and glow sticks.

He even claimed to find an foetus inside a dead cow, which was like a “cross between a human, a monkey and a frog" and speculated on radio the cows were being used as incubators for alien babies.

The late Paul Bennewitz, who had a PhD in physics, was researching classified government programs at Kirtland Air Force base in Albuquerque, New Mexico, during the seventies.

He picked up electronic signals near Dulce, and theorised they were coming from an underground base, before he suggested the Dulce theory in 1982.

The late Philip Schneider was next to ramp up the myth.

He went whistleblower to claim he survived an extraordinary battle at Dulce in 1979 and said he was an explosives engineer who worked for the US Government under high-level security clearance, to finish building the base using monster drills.

Schneider claimed that during drilling he came face-to-face with a seven-feet tall “grey alien.”

In a tale from an alien horror flick, he said he shot and killed two aliens, but was blasted with a plasma ball that blew off some fingers.

He was saved by a US Green Beret, and in a battle that followed 57 people perished and three survived, including Mr Schneider, he claimed.

Many people have sought to debunk Mr Schneider’s claims, saying he claimed to work for the government for 17 years, but retired at 34, so would have started work at 17.

A former roommate also came forward to say Mr Schneider previously told them he lost his fingers working in eastern Oregon as a lineman.

The only other alleged “witness” to emerge was supposedly called Thomas Castello, an alleged security guard at the base.

He said he had seen people reported as missing locally taken to be experimented on, in the run up to the battle.

And his account was linked to purportedly leaked images of people held inside strange water-filled bags, and other so-called Dulce Base pictures which appeared online.

The images were later proved as hoaxes, taken from the Arnold Schwarzenegger film The 6th Day, released in 2000.

An article on Theblackvault.com, which exposed the images as hoaxed, said: ”Multiple online sources and viral posts, have circulated this photograph as having been leaked by Costello, then shortly after it was leaking, he and his family went missing. 

“There is no evidence that I have found, that shows that Thomas Costello even exists.” 

From 1988 to 1990 US Ufologist John Lear claimed to received “independent confirmation” that the Dulce base existed.

Writer Mike Rothschild investigated the myth for sceptical website skeptoid.com.

He said: “There are no real pictures of it. There is no physical evidence of roads or vents or doors or anything of the sort. Even though tens of thousands of people must have been involved in building, staffing and guarding the base, nobody has ever claimed with any credibility to have worked there or that their family member or friend was killed in a shootout or experimented upon.

“I can’t conclusively prove Dulce Base doesn’t exist. 

“But nobody can conclusively prove it does. Until that proof emerges from the ground, Dulce will remain a strange story created by troubled dreamers, not a house of alien horrors.”

Political scientist Michael Barkun said Cold War underground missile installations dug out in the area gave “superficial plausibility” to the Dulce base myth.

He said claims about experiments on abductees and the battle put the Dulce legend "well outside even the most far-fetched reports of secret underground bases."

Despite this, the area remains a hot bed of UFO activity, according to locals, which may well keep the myth alive.