This ancient civilization may have hit the nail on the head. It's very well possible the world will end 12-21-2012.
The Mayans had a calander that scientists say was amazingly accurate and could have only been based on astronomical movements.
But, the most interesting thing about the calender was its placement of the time of the Mayan civilization. The calendar started centuries before the Mayan civilization but the calendar ends on 12-21-2012 for no apparent reason.
Scientists have suggested that there are two possible reasons for the calender running out. Some hypothesize that the calendar was not supposed to be interpreted as liner, buy cyclic with the intent of starting over from the beginning. But that is not very likely because specific events are noted(Beginning of the Mayan civilization). The second hypothesis is that it predicts the end of the world...but does not indicate from what.
The most likely end of the world scenario is another mass extinction caused by a collision with an asteroid. Scientists state that this is a matter of when not if, inasmuch as it has happened several times before. And given the accuracy of the calendar, it gives pause for thought.