Yellowstone National Park
Yellowstone National Park in Wyoming is famous for its hot springs and geysers. This is because it's sitting atop the worlds biggest volcano, which has a habit of erupting every 650,000 years or so. And we are long overdue for the next eruption. Such a catastrophe would fill the entire atmosphere with ash, blotting out the sun around the world and plunging Earth into a 15,000-year winter.
Magnetic Field Reversal
Our planet is surrounded by a magnetic field that protects us from a lot of the sun's radiation. It undergoes a massive reversal - north switching poles with south - on average every 250,000 years. Right now we're about 30,000 years over that average. Significantly, the poles are rapidly drifting apart by 30kms each year, pointing to an imminent reversal (feasibly in 3000 - 4000 AD). Mayan enthusiasts reckon it may be sooner than we think. This disruption will cause the magnetic field to disappear entirely for up to a hundred years, leaving us with the kind of exposure to the sun's UV light that would turn you to a crisp in a matter of seconds.
Meteor Impact
433 Eros, named after the Greek god of love, is one of the few Near Earth Asteroids (NEAs) over 10km wide. In fact, it is thought to be larger than the impactor that created the Chicxulub Crater in Mexico, which has been linked (at least in part) to the extinction of the dinosaurs. Eros is calculated to pass by Earth at 70 lunar distances in 2012, although there is always room for error.
The Large Hadron Collider
While physicists are smashing atoms in Switzerland in the hope of finding The God Particle, laymen expect it to create a black hole that will destroy Earth. In truth, scientists at the atom smashers in America have been creating mini black holes this way long before the LHC was switched on. Their smaller colliders can cause black holes to pop in and out of existence in a fraction of a second. Nonetheless, fear mongers insist that in 2012 the Large Hadron Collider will crush Earth into the size of a marble.Update: In August 2009 on my trip home to Europe, we took a free tour around CERN's Large Hadron Collider. I can't quite convey the sheer size of this experiment; the collider itself spans a 27km track, which must be cooled to temperatures of -270 degrees Celsius to operate. The particles are accelerated using powerful magnets and finally smash together inside a complex detector. Smaller bits of energy fly out in all directions, but even the whizziest ones (muons) run out of steam before they reach the surface some 100 meters up. I should add that we are constantly bombarded by cosmic rays like this from outer space. When we asked our tour guide (one of the engineers at CERN) "when are you going to destroy the Earth with a black hole?" he just laughed incredulously. Methinks the laymen should stop jumping to conclusions.
So now that I've got all the doomsday theories out of my system - let's get back to the original Mayan Prophecy. It's all about a change in global consciousness; a maturing of minds. Some believe this will be sparked by a catastrophic event, like a global economic collapse that takes us back to living off the land. Others suggest it may be a cultural thing, as more people become aware of their spiritual roots and become less materialistic. Others yet believe that the Mayan calendar is fundamentally flawed, because it's an arbitrary date system that uses base 10 (as quantum physicist Brain Cox points out, what if then Mayans had 12 fingers?).
Either way, speculating over what will happen in 2012 probably won't do us any good. There will always be natural and man-made threats and we should be reasonably prepared for these. But acting like the world is going to end won't help you enjoy life today. Bizarrely though, the immense popularity of the Mayan Prophecy may be enough to raise the global consciousness and set our money-driven society on a new path of happiness and enlightenment today.