I don't know how people can think that some cataclysmic event is seriously going to happen on Dec 21, 2012. I mean the 'Mayan' calendar isn't even that accurate of a calendar. I think I read somewhere it had an error rate of 1 day for every 4 years. The Gregorian calendar was actually far better 1 day to 3000 years.
Anyways, defining the 'end of the world' as the total loss or irreversible sequential demise of the human race, a natural cause would seem to be out of the question natural disaters always have signs leading up to them.
Meteor large enough to wipe out all life on the planet? While not as extensive nor effective as the NASA would like at the moment, there is a detection capablilty and something that big we would have seen already.
Super-volcano eruption? Way too many signs we would be seeing now if something like that was imminent.
Rogue black-hole? Again we would be seeing it effects already.
Plague? Sure, a possiblity to happen something that will most likely happen again, but there has never been a plague in the entire span of recorded human history that was capable of wiping us all out.
Zombies? I have my shotgun and baseball bat, not happening.
That leaves man-made, and those options are really limited. In fact, I can only think of the obvious one of nuclear war and the current geo-political clime doesn't exactly show that happening.
Edited by RightInTheStatisticals, 02 May 2012 - 09:43 PM.