The whole 'Mayan Doomsday Calendar' is an amalgam of all sorts of older doomsday theories combined with (what I have read in scientific debunkings of the scenario) an at-best loose understanding of Mayan civilization, combined with Aztec and other civilizations around Central and even South America- as it seems to get them all confused, yet still label it Mayan.
The prediction itself is based on ludicrous math and reasoning that no other scientist or skeptic has been able to replicate or follow, without already 'assuming/believing' that it's true from the beginning. Nothing will happen, except all the people who bought items with "no down payment until 2012" are really gonna be hurting afterwards.
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My personal explanation is; SCENE, A man is carving out a large calender, when his best friend walks in, looking excited:
"Hey how's it going? How far are you?"
"Pretty good, just finished off 600 years"
"yea yea yea thats cool, now comon, there's some cool people on horses with metal helmets that just arrived! You can finish that later, lets go!"