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There are three possibilities. Doomsayers believe December 21, 2012, marks the end of an ancient Mayan calendar - and, therefore, the onset of world-ending natural or extraterrestrial catastrophes.
A hitherto unknown Planet X may crash into the Earth that day, or aliens may invade. The threats of physical destruction are numerous.
"I'm going underground on the 19th and coming out on the 23rd. It's just in case anybody's right," said Ron Hubbard, an American manufacturer of high-tech survival bunkers.
Mr Hubbard told the Daily Telegraph in England that his business has been doing well as the famous date approaches.
"I've gone from selling one a month to one a day," he said.
"I don't have an opinion on the Mayan calendar but ... astrophysicists come to me, buy my shelters and tell me to be prepared for solar flares, radiation, EMPs [electromagnetic pulses]."
There is a scramble on as many people take action "just in case".
In China, this has meant stockpiling candles, apparently considered a formidable countermeasure against apocalypse.
One grocery store owner said: "At first, we had no idea why. But then we heard someone muttering about the continuous darkness."
The tendency is also strong in Russia, where survival kits have been sold with the motto: "It can't be worse".
The proponents for this idea of the Earth's destruction have for many years been collecting scraps of archaeological and astronomical information and hearsay to piece into intricate theories.
They start with the Mayan "Long Count" calendar, which contains "Great Cycles" lasting 5125 years.
In one interpretation of Mayan literature, there have been three prior worlds, each of which ended after a Great Cycle.
The end of the fourth world - our current one - is to happen on December 21, 2012, and it is seen by some to be final.
Archaeological evidence includes Monument Six, a stone tablet found in an obscure ruin in southern Mexico in the 1960s during highway construction.
The site was paved over and bits of the tablet looted - but a partial inscription was able to be recovered.
It said something was to happen in 2012 involving a mysterious Mayan god of war and creation, Bolon Yokte... But the description of what would actually happen had been rendered virtually unreadable by a crack and erosion.
Guillermo Bernal, an archaeologist at Mexico's National Autonomous University, believes the lost message is: "He will descend from the sky."
Others have filled in the blank with a theory of galactic alignment. On the 21st, Earth will fall in line with the centre of the Milky Way galaxy - and a supermassive Black Hole, the hypothesis goes.
Gravity could be wildly disrupted, or a blast of cosmic rays force millions of years of evolution to happen in just three days, supposedly. In case the destruction is partial, people have been advised to be ready for survival with standard emergency kits.
But not everyone is impressed by the widespread musings about December 21. Nasa in particular has long battled the idea of a prophesied apocalypse.
"Doomsday is not real, but people's fear promoted by countless websites and YouTube videos is unfortunately very real, and especially harmful for children," the space agency has said.
Other public statements have humourlessly discounted as "just plain silly" - or even "evil" - the notions of rogue planets careening towards Earth or galaxies falling into a grand alignment.
"None of this has anything to do with the galactic equator or any of the other nonsense about alignments that appears on many of the conspiracy theory websites," Nasa said in response to one pseudo-scientific theory.
A writer at the agency, apparently paranoid about an impending mass hysteria, posted online: "I hope that most people are able to distinguish Hollywood film plots from reality."
Even more temperamentally, websites have sprung specifically to fight doomsaying troublemakers.
The sites are loaded with red graphics, panicked prose, bold type and bold-and-underlined type, but Nasa nevertheless refers people to them as useful resources.
A particularly irascible website, 2012hoax.org, says: "The '2012 doomsday' is a hoax, a fraud, and an absolute con job. It is a cruel and disgusting lie being promoted by scam artists after money; first they scare people to death that something terrible is going to happen, then publish books and videos on 'how to survive the apocalypse'."
The second possibility for December 21 is that it will simply be the dawn of a New Age of consciousness.
Mayan leaders have come out saying the end of the calendar is not meant to be the end of the world - just a new beginning. Pedro Celestino Yac Noj, a Mayan sage, has said: "The 21st is for giving thanks and gratitude and the 22nd welcomes the new cycle, a new dawn."
More ambitious mystical traditions have emerged on this basis. They profess December 21 will bring a mass awakening for humanity.
"At any end-beginning nexus - at the dawn of a new religion or a spiritual tradition - you have this amazing opening," Mayan scholar John Major Jenkins told the New York Times.
"Revelations come down. There's a fresh awareness of what it means to be alive in the full light of history."
The director of a group calling itself the Metaphysical Research Society told the newspaper: "To me it's all about a movement toward enlightenment. We say compassion over competition.
"This whole shift in consciousness is going to wipe away everything negative. Armageddon isn't what it used to be, you know?"
Jose Arguelles, among the most prominent leaders of such movements, argued the Earth would become permanently wrapped in a rainbow as we entered the new world.
He was said to be living in New Zealand to prepare humanity for its shift into galactic consciousness, but some reports say he died last year.
The third possibility is that the end of the Mayan calendar may simply be the sudden end of all time and existence, with no further explanations necessary.
The top 5 scenarios for the end of the world as we know it
1) Aliens
Remember Independence Day? Astrophysicist Craig Kasnov recently announced that three very large, fast-moving objects were approaching Earth. Though largely discounted by others, the SETI (Search for Extraterrestrial Intelligence) researcher Kasnov said the "flying saucers" were tens of kilometres long and would arrive mid-December.
How to prepare: Some say you should head to the French Pyrenees, from where you may be beamed up and rescued.
2) Natural disaster
Supervolcanoes cause mass extinction. The most recent occurred in New Zealand about 26,000 years ago, while a likely site for the next eruption is Yellowstone National Park, in the United States.
How to prepare: Leave the Pacific's Ring of Fire, preferably for equatorial Africa, where humans survived the last volcanic winter.
3) Nuclear attack
Surviving the initial horror of thermonuclear war is just the beginning. A nuclear winter will follow as debris, smoke and soot block sunlight for weeks or even years, and then there's radiation sickness, lawlessness and the breakdown of civilisation to worry about.
How to prepare: For a quick end, stand outside as the bombs fall. Otherwise, build an underground bunker with enough supplies, guns, clothing, etc, to survive in the Mad Max-style post-apocalyptic wasteland.
4) The god of war cometh
Bolon Yokte, a Mayan deity, is prophesied to return to Earth in 2012, although key passages on a stone tablet containing the prophesy have eroded away.
The god may cause huge chaos and upheaval - or bestow upon people the energy to take an evolutionary step.
How to prepare: Only human sacrifice can save us now.
5) Salvation
Biblical messianic prophesies suggest a Second Coming of Jesus, possibly including resurrection of the dead. Armageddon is often associated with such an event.
How to prepare: Watch Supernatural from series three onwards. And pray a lot.
Best places to experience the End
Pitt Island
On this eastern New Zealand territory, which famously sees the first New Year's sunrise, you'll reach the fated day early. Enjoy a long December 21 while the rest of the world - and its lagging time zones - catches up to the date. You may die first, however.
Fly non-stop from Newark to Singapore.
At 18 hours 50 minutes, the flight will keep you off land for most of the apocalyptic day. And because it crosses the international date line, you should be able to all but miss the 21st altogether.
The Maldives
It's considered by many to be paradise on Earth, and it's also a long way from seismic activity in the Pacific. Chances are you'll have a good time.
Nervous? Recommended remedies
Lifeline
Counselling available on 0800-543-354. No prank calls please.
Rescue remedy
A combination of flower materials said to ease panic attacks and stress.
Recall the smile of a loved one
A Guardian investigation into calming your nerves before a job interview suggested this mind game. It should apply.
So long, and thanks for all the fish
DO: Swim with dolphins.
As a bonus, dolphins are hyper-intelligent beings who may guide you to salvation (or, as per The Hitchhiker's Guide to the Galaxy, leave you stranded after thanking you for all the fish).
DON'T: Commit a crime.
Police are expected to keep arresting offenders till the last moments, and you don't want to spend them in jail. In addition, the world might not end.
DO: Meditate.
Several schools of meditation consider its ultimate goal is to reach Nirvana or paradise, a state of being free from the world's shackles. Transcend reality and avoid the apocalypse.
DO: Practise lighting fires.
Imagine a scenario in which the end of the world is half-hearted. The planet is devastated, but you're still alive. Survival skills will be handy, starting with: howtostartafirewithoutmatches.com
DON'T: Panic.
Survival gear - a checklist:
Underground bunkers
Why: Designed just for these occasions.
Where to get one: The Vivos network offers a range of US-based bunkers accommodating 10 to 2000 people.
Price: Adults $50,000, sprogs half-price.
Scuba equipment
Why: Surging seas feature in several scenarios. Don't count on rentals.
Where to get it: Dive retailers.
Price: $1500 for a set deal.
Hazmat suits
Why: Protection in case of nuclear war, solar storms or alien blasters.
Where to get one: www.radshield.com
Price: $1700 for a full-body suit.
Heavy artillery
Why: To fight aliens and asteroids on the day, or roving gangs post-apocalypse.
Where to get it: gunsamerica.com or, from Iran, diomil.ir
Price: $13,500 for an anti-tank gun.
Bruce Willis and Sigourney Weaver
Why: A OnePoll survey found the actors were the top choice for saving the world.
Where to get them:
Los Angeles, CA 90069
Price: About $20 million (reportedly) for Willis to play hero.
Matches
Why: In case of survival, you'll need to cook food, boil water and keep warm.
Where to get them: The dairy.
Price: $1 or so.
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