On December 21, 2012, the apocalypse foretold 5,125 years ago by the ancient Mayans will come to pass and the world will end.

Meanwhile, Harold Camping, an American radio preacher, got thousands of followers worked up when he predicted the Second Coming of Jesus Christ on May 21 last year.

When that didn’t happen, Harold Camping said the world would end on October 21. And then he quietly retired from his radio show.

But the “2012 phenomenon”, as it is commonly known to its legions of internet followers, is different.

For the Mayans, a famously wise and advanced civilization which was at its height between 250 and 900AD in the present-day Mexican state of Yucatan and Guatemala, have grabbed everyone’s attention.

The evidence boils down to one simple fact: their 5,125-year calendar – the one used across Central America before the arrival of Europeans – runs out on December 21 this year.

The point is that the Mayans were noted for their extraordinary astronomical observations and mathematical powers.

And if they didn’t think it worth taking their calendar beyond December 2012, they must have had a reason.

Public concern is so high that NASA, the U.S. space agency, even has a section debunking the theories of impending doom on its website.

NASA says it has taken more than 5,000 questions from people, some asking if they should kill themselves, their families or their pets.

Archaeologists who have studied the Mayans have been downplaying the apocalypse theories, insisting that the only surviving Mayan reference to any dreadful significance attached to December 21, 2012, was contained on a single ancient stone tablet found at ruins in Tortuguero, southern Mexico, in the 1960s.

Mexico’s tourism agency hopes the 2012 phenomenon will draw 52 million visitors to the region - more than twice the number the whole country normally receives

According to an inscription on the tablet, a fearsome Mayan god of war and creation may “descend” from the sky on the appointed day.

But then, a few weeks ago, archaeologists had to admit they had found a second piece of evidence – a 1,300-year-old carved brick fragment at a temple ruin in nearby Comalcalco.

The brick, now kept in a vault at Mexico’s National Institute of Anthropology and History, has an inscription on its face which also refers to the date.

The fact that the face of the brick was probably laid facing inward or covered with stucco – suggesting it was not meant to be seen by the Mayan population who visited the temple – has only added to the hysteria of modern doom-mongers.

Scientists insist there is no dire threat on the horizon, while Mayan experts stress that the ancient civilization’s legacy has simply been misinterpreted.

“Nothing bad will happen to the Earth in 2012,” says NASA on its website in the reassuring tones of a parent dealing with a frightened toddler.

“Our planet has been getting along just fine for more than four billion years, and credible scientists worldwide know of no threat associated with 2012.”

Of course in these conspiracy-obsessed times, there are thousands of cynics who are not convinced.

David Morrison, senior scientist at the NASA Astrobiology Institute, said he had been receiving about ten emails a day from worried members of the public who are “seriously, seriously upset”.

A young woman from Denmark wrote to him saying: “Mother of one daughter and another coming.

“Yesterday I was considering killing myself, the baby in my stomach and my beloved two-year-old daughter before December 2012 for fear of having to experience the Earth’s destruction.”

A 13-year-old American, wrote: “I am considering suicide. I am scared to tears . . . I don’t want to live any more, I deserve an explanation.”

Another wrote: “I am so scared. My only friend is my little dog. When should I put her to sleep so she won’t suffer when the Earth is destroyed?”

Worried Americans are rushing to buy everything from $27 survival guides to $50,000-per-person places in bunkers that are marketed as being both nuclear bomb and asteroid-proof.

Robert Vicino, a Californian businessman who is building the luxury bunkers in secret locations asked on his website: “What if the prophecies are true? Which side of the door do you want to be on?”

The businessman says that he has more than 5,000 Americans booking places, and is now building bunkers in Europe.

Steve Cramer, one man who has reserved his place, insists: “We’re not crazy people: these are fearful times. My family wants to survive. You have to be prepared.”

Jason Hodge, a father-of-four who also counts himself a “future survivor”, to use the jargon of the apocalypse industry, adds: “It’s an investment in life.

“I want to make sure I have a place I can take me and my family if that worst-case scenario were to happen.”

Mayan apocalypse converts have started flocking to Bugarach too, a tiny hilltop town in the foothills of the Pyrenees.

The 200-strong local community has had to contend with 20,000 visitors since the start of last year, and the French government is worried about the threat of mass suicides.

Believers say a magnetic force surrounds the town’s “mystical” mountain where the top layers of rock are older than the lower ones.

(Geologists say that soon after the mountain was formed, it exploded and the top flew into the air, before landing upside-down).

People claim the magnetic force will protect them from the apocalypse to come.

Others who have flocked to Bugarach insist the mountain is a gateway to another dimension and may contain a secret alien base.

Unhelpfully, the Mayans did not specify exactly what would happen when the world ends. But that hasn’t stopped believers from letting their imaginations run riot.

Many of their 2012 doomsday scenarios involve astronomical phenomena – a rogue planet hitting Earth, fierce solar storms, a galactic alignment in which the Sun’s gravitational effect combines with that of a huge black hole to create havoc. The gloomiest think we may get all three.

A particularly popular theory is that a rogue planet called Nibiru is lurking behind the Sun and will collide with the Earth next December, destroying it. Some believe this rogue body is Eris, a dwarf planet orbiting beyond Neptune.

The idea of a planet creeping out from behind the Sun and smashing into Earth provided the depressing backdrop to last year’s Lars von Trier film Melancholia, starring Kirsten Dunst and Kiefer Sutherland.

Another theory, also involving the Sun, predicts that a huge solar flare – called a “solar max” – will destroy the Earth.

This notion has already inspired Hollywood in the 2009 disaster blockbuster 2012, in which the flare caused catastrophic earthquakes. The film also made reference to the Mayan calendar.

Finally, no apocalypse would be complete without at least one alien invasion.

This time last year, reports emerged suggesting the U.S. Search For Extraterrestrial Intelligence Institute (SETI) had detected three large spacecraft due to arrive at Earth in 2012. SETI rejected the claims, to which those who wanted to believe the reports replied: “Well they would, wouldn’t they?”

Another alien theory doing the rounds among conspiracy theorists is that the authorities will stage a fake extra-terrestrial invasion at next year’s closing ceremony for the London Olympic Games so they can declare martial law and introduce a new world order.

Academics and scientists dismiss all of these theories as wild hysteria, of course.

But the fact is that Mayan scholars have been bickering for years over what the end of the Long Count Calendar actually signifies.

The Mayan calendar began in 3,114  BC – believed by Mayans to be when the current “world order” was created – and progresses in 144,000-day cycles (a little more than 394 years) known as baktuns.

The 13th (a sacred number for Mayans) baktun runs out on the 2012 winter solstice, December 21. After that date, the “Great Cycle” is completed and the calendar sequence simply ends.

In 1957, respected Mayan scholar and astronomer Maud Worcester Makemson wrote that the completion of a “Great Period” of 13 baktuns would have been “of the utmost significance to the Maya”.

In 1966, Michael Coe, another prominent Mayan anthropologist and a former CIA agent, went much further and concluded there was a “suggestion” among the Mayans that the final day of the Great Cycle would see “Armageddon overtake the degenerate peoples of the world and all creation” and “thus . . . our present universe would be annihilated”.

Experts had tended to agree with Michael Coe’s interpretation until about a decade ago when the academic world started to insist the Mayans had meant nothing of the sort.

The Mayans believed the end of the 13th baktun would indeed be significant, say academics now, but in a good way.

There will simply be another cycle and it will be a cause for celebration not desperation.

This optimistic message has been championed by many in the New Age movement, which is obsessed by the idea that cultures such as the Mayans had a secret spiritual knowledge that we might tap into if only we knew where to look.

Whatever the truth, hundreds of books have already been published on the subject, not to mention dozens of television programmes and films.

For the most reliable indication of the future, we should perhaps head for the heart of Mayan territory in south-eastern Mexico.

There, locals aren’t running for the hills at all, and don’t seem worried.

In fact, quite the reverse. After suffering years of a tourist industry badly hit by the violence of warring drug cartels, they are looking forward to an economic boom.

Mexico’s tourism agency hopes the 2012 phenomenon will draw 52 million visitors to the region – more than twice the number the whole country normally receives.

And the town of Tapachula, on the Guatemalan border, has already started a countdown to December 21 on a giant digital clock in its main park.

Quite what will happen on the day it runs out remains the subject of feverish debate around the world.