Have you guys heard of the shift in the Earth's magnetic field?
According to astrologists there will be a period when the North magnetic pole will become South, and the South become North, due to an inversion of the currents in the Earth's core.
Besides all the fun minor implications (all compasses starting to point South, sink-water spiralling down the drain in an anti-clockwise motion instead of a clockwise motion [if you live in the North hemisphere][or is it the other way around?]), there will be an extremely short period (on an astronomical scale, that is a few months) where the Earth will have no magnetic field. If such a thing happens, the only protection we'll have against incoming meteorites will be solar winds.
Of course, it's a rare event on a human scale, as the last one we know about happened 780000 years ago.
As to when that period will be, "we" aren't sure, but "we" think it'll happen during this century. The exact date is hard to determin because this requires very precise measurements.
Which is why some people turned to the Mayan calendar, which is a lot more precise than ours.
Basing themselves on the fact that the ending of a cycle corresponds to a major astronomical event, they discovered that December the 21st of 2012 was an end of a type of cycle in the Mayan calander (I say a type of cycle because there are LOADS of types of cycles in the Mayan calendar), and making a correlation between this and that decided it would either be the beginning of a new world, or the end of this one, or peace on earth, etc. In other words, the Apocalypse. As for if I believe what'll happen, I don't know. If we don't have any comet protection for 3 months, it'll get rid of a lot of things, unless overpolluting the planet catches up with us first.
