Late at night, on October 4 2002, a strange guest appeared on a cult American radio show. Coast to Coast AM with Art Bell had a reputation for exploring weird themes with fascinating guests, but few had ever sounded as excited as this one.

Dallas Thompson was a former personal trainer who had spent his youth in Hawaii but now lived in Bakersfield, California. His life had changed forever following a terrible accident, five years earlier. He’d been driving along Highway 58 during heavy rain when his car had aquaplaned, spinning four times, only to plunge backwards down a 250ft drop.

When Thompson was found, the roof of his blue Honda Accord had been crushed almost to the floor. The fireman who rescued him was amazed he hadn’t been decapitated. As he’d been sitting, helpless, in the wreck, Thompson had had a vivid near-death experience. He claimed to have seen a “light so bright that it burnt my eyes” and made him “legally blind” and to have had bizarre knowledge about the world poured into him. When he regained consciousness, he was convinced that the Earth was hollow and had an opening at the North Pole. He’d come on Coast to Coast to discuss his mission to locate and explore it.

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“There are cavern systems and caves that traverse the whole mantle,” he told Bell, whose scepticism often took the form of slightly extended silences. Because of the special atmosphere in the hole, Thompson explained, living creatures were protected from pollutants and harmful rays. There were herds of mammoth and ancient tribes down there, the members of which lived to be around 1,700 years old.

“How do you know all this?” asked Bell.

“I just do,” said Thompson. “I remembered stuff that has been forgotten.”

Later, Bell asked after his mental health.

“Are you manic?”

“I’m just excited,” said Thompson.

“… I can tell.”

Perhaps most incredibly, Thompson revealed he’d secured funding to travel to the hole with a helicopter backpack called a SoloTrek, which he’d use to descend into it. He even had a date for the trip: May 24 2003.

Over the next few months, news of Thompson’s expedition spread. He began to receive emails from media companies keen to report the story and many more from both critics and admirers. The sprawling book he’d written, which included his theories about Hollow Earth, began to sell.

In December 2002, two months after his radio appearance, he posted a message on his Yahoo Group page describing an inundation of “over 5,600 emails every few days”. He said his book, Cosmic Manuscript, had become a bestseller but he was pulling it from sale. “I have requested the book be discontinued even though it’s still at the top of the charts in Canada,” he wrote.

And then, the most mysterious event of all took place. All of a sudden, Thompson disappeared…

For many centuries, humankind has dreamt of inner worlds. Numerous writers have been inspired by the idea, not least Tarzan creator Edgar Rice Burroughs (At the Earth’s Core, 1914), Edgar Allen Poe (The Narrative of Arthur Gordon Pym of Nantucket, 1838) and, most famously, Jules Verne, whose 1864 novel A Journey to the Centre of the Earth has been adapted numerous times for both stage and screen.

Thousands of contemporary theorists discuss what is commonly referred to as “the greatest ever secret” on internet forums. On YouTube, videos claim that the satellite images on Google Earth have been altered to cover up the existence of the holes, while other videos claim the holes are there for all to see.

Ancient Greeks, Tibetan Buddhists and Christians all located their iterations of hell in caverns under the mantle. Pilgrims used to journey to Station Island off the coast of Co Donegal where they believed there existed an entrance to Purgatory, while the old legends of Mexico describe a mountain cave near Ojinago inhabited by devilish creatures from “the way-down-deeps”.

The 18th-century Astronomer Royal Edmond Halley believed that unusual compass readings could be explained by the fact that the planet was composed of a hollow shell, two inner concentric shells and an innermost core about the diameters of the planets Venus, Mars and Mercury respectively.

One of the most famous Hollow Earth theorists, and a true predecessor of Thompson, was a veteran of the 1812 Anglo-American war, John Symmes. In his book Banvard’s Folly, Paul Collins recounts the “theory of concentric spheres and polar voids” that preoccupied the soldier.

Symmes published a pamphlet, in which he wrote, “I declare that the Earth is hollow and habitable within; containing a number of solid concentric spheres, one within the other, and that it is open at the poles 12 or 16 degrees.” He pledged his life to promoting his notion, boldly declaring, “I am ready to explore the hollow.”

He toured the US with a handmade wooden globe that opened out to reveal its secret layers. Converts, in ever increasing numbers, began petitioning the government to finance his adventures. On March 7 1822 Senator Richard Thompson presented a case to Congress that Symmes be supplied with “the equipment of two vessels of 250 to 300 tons for the expedition, and the granting of such other aid as Government may deem requisite”.

During the debate, it was suggested that the Committee for Foreign Relations become involved, as the trip may well bring Symmes and his crew into contact with new races of interior people. But the motion was to fail. Seven further bills were presented to the House. Not one succeeded.

Symmes spent the rest of his life lecturing and lobbying for action. “In May 1829,” writes Collins, “Symmes died, believing right up to the end that the greatest discovery in human history had eluded his grasp.”

But still, today, men grasp. One of them is Rodney M Cluff, author of World Top Secret: Our Earth IS Hollow! “I was working on a New Mexico farm when I was 16 and the farm manager’s son started talking about it,” he says. Fascinated, he began reading up. “I found evidence from the Scriptures, history and science that our Earth is hollow as well as all the planets and the moons and even asteroids.”

So convinced was Cluff that, in 1981, he flew his wife and five children from New Mexico to a new life in Alaska. “I thought, ‘Why don’t we see if we can find the way to the Hollow Earth?’ ”

_{A 'map' of the Inner Earth}

And was his wife keen? “She wanted to go back home. She thought I was crazy. But we did it anyway.”

In Alaska, Cluff met a small group of people who had travelled to the icy state with the same idea. Soon they were ready to embark upon their mission. “We started on the road up to Point Barrow,” he says. “We saw a sign, at one point, saying ‘This Is A Private Road: Don’t Go Any Further’. So we didn’t go any further.”

How long did he drive before he reached the sign and aborted the mission? “About an hour,” he says. There’s a silence while I process this information. “Less than…” Soon afterwards, the Cluff family moved back south.

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Like many believers, Cluff is convinced by the accounts of others who claim to have already found and even visited the “inner Earth”. Chief among them is Karl Unger, a German sailor said to be part of a 1943 U-boat expedition to the South Pole. The submarine apparently entered the Hollow Earth through an underwater passageway, and its crew were greeted by an advanced civilisation in a place called “Rainbow Island”. (Hitler was reportedly a believer, and some conspiracy theorists are convinced he escaped to the Hollow Earth at the end of the Second World War and is still there.) Then there’s Admiral Richard Byrd, a highly decorated US Naval officer whose supposedly hushed-up “secret diary” of a 1947 expedition to the North Pole is believed to contain descriptions of a land full of lush lakes, greenery and woolly mammoths (Byrd was actually in the South Pole at the time). There’s even a retired colonel in the US Air Force, Billie Faye Woodward, who claims that he and his twin sister (both hermaphrodites) were born in the Hollow Earth.

Cluff, for his part, tried to reach the Hollow Earth again. In 2003, he received an email from a man named Steve Currey who’d recently inherited his family’s travel firm that specialised in far-flung expeditions. Currey had once heard his father talking about the Hollow Earth and was familiar with Cluff’s book. They decided to plan a new trip.

“We worked on it for several years,” says Cluff. The scheme involved chartering a Russian nuclear ice breaker that was used to take tourists to the North Pole. Once the basics were worked out, they began recruiting members. “Steve was charging about $26,000 for a spot on the ship and he actually got about 40 people to put down the money.”

Before the voyage, they chartered a plane to fly over the pole to locate the opening. “We were going to leave in August 2006. But in April of that year, Steve found out he had six inoperable brain tumours. Just before we were ready to fly, he died.”

Another member of the expedition – Dr Brooks Agnew – was appointed as the new leader. After renaming the operation “The North Pole Inner Earth Expedition” and raising yet more funding, they planned for a summer 2014 departure. But a further unexpected disaster befell the team.

_{Hollow Earth believer John Symmes, right, and the 'hole at the pole' (Mary Evans Picture Library / Alamy)}

“Brooks Agnew resigned last September,” says Cluff. “He said a major stockholder in his company had withdrawn all their money, saying it was because [Agnew] was involved in an expedition to find the Hollow Earth.”

When another key member of the team died in an aeroplane crash, Cluff began to wonder if mysterious powers were manoeuvring against them.

“There seems to be some force that’s trying to stop this happening,” he says. “I think it’s the international bankers. They don’t want the Inner Earth people messing around with their slaves, here on the outer world.”

And what about Thompson? His final posting in his Yahoo Group page had been on January 11 2003. Then, he had vanished. Adherents of Hollow Earth theory, writing in the chat forum, couldn’t help but speculate. “Maybe there is something someone did not want him to find,” said one.

“It is quite a mystery,” said another, before wondering if he had made his trip north. “Maybe he’s there?”

Further investigation, however, suggested a rather more prosaic answer. Thompson’s book, Cosmic Manuscript, is still available to buy on the internet. Among all the five-star reviews (“I am a doctor who has read thousands of books, and Cosmic Manuscript is in a class of it’s own”; “I consider there are no accidents. This book came to me instead of me searching for it”; “I LOVE this Cosmic Manuscript”) there’s a solitary one-star review.

“Sorry, babbling moon bats,” begins its author, a W M Mott. “Material in this ‘book’ was plagiarised, lifted, STOLEN, word-for-word, from previously published materials. The ‘author’ is IN HIDING as a result, and refuses to surface for fear of well-deserved lawsuits.”

Mott goes on to allege that Thompson “lifted entire pages and paragraphs from my earlier book, Caverns, Cauldrons and Concealed Creatures”. These were serious allegations. Could this be the truth behind Thompson’s evaporation?

When I contacted Mott, he told me, “I have good reason to believe that I have located Mr Thompson. He is not blind, nor has he ever been. In fact, he is passing himself off as a writer again.” Mott supplied a link to the self-published web-biography of an individual named Dallas W Thompson. This individual lived in the correct place, Bakersfield, California. Like our man, he’d spent a portion of his youth in Hawaii. He was also the author of a self-published book – a thriller called Eyes Wide Shut. A photograph shows a heavyset older man in a peach shirt staring directly in the camera in a way that strongly suggests someone who is not blind. But when I contacted him to request an interview about the Hollow Earth, he emailed back, “The Dallas Thompson you need to talk to is not me.”

It sounded unlikely. There were just too many coincidences. Nevertheless, further digging uncovered the original local newspaper cutting that reported Thompson’s car crash. It named him as Steven Thompson and revealed that he was 26 when the accident happened in 1997. But the biography of the Dallas W Thompson that W M Mott had accused of plagiarism had said he was born in 1944, a significant difference. Indeed, the photograph on his website shows a man who appears to be in his 60s.

Perhaps there are two Dallas Thompsons in Bakersfield?

Eventually, I find a phone number that must surely belong to our man. He lives in Bakersfield. His name is Steven D Thompson. His age, estimated by a records database, is quoted as between 40 and 44. Right place, right name, right age. Excited, I ring the number. It’s disconnected.

Maybe Thompson is in hiding. Maybe government forces or evil bankers made him disappear, terrified of the world-changing truths he was about to unleash. Maybe he did journey to Hollow Earth, descend into it with his helicopter backpack and is now prancing joyfully with the mammoths, and the ancient tribes, living in a paradise of pure air, warm climes and abundant food that will sustain him for another 1,657 years.

Or maybe he forgot to pay his phone bill.

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