This being a very controversial issue, I'm sure everybody has their own opinions and predictions for perhaps the most famed December in history. Many believe it will bring with it worldwide destruction; others believe it to be a hoax, a lie. I believe neither. Something most likely WILL happen, but it will definitely NOT destroy the planet or end humanity.
The Mayan calendar is a cycle. That's why it is represented by that giant circular slab of rock. But that's only one of the Mayan calendars. In total, there are around 23, all interlocking into various cycles and patterns. The one we are concerned with is the "Long Count" calendar, which measures a period of around 5120 years. This calendar ends its current cycle on - you guessed it - December 21, 2012. This is where people get all the doom and gloom from (along with the Aztec calendar, which was basically stolen from the Mayans). But don't worry guys; the Mayan calendar doesn't end on 12/21/12 for the same reason our calendar doesn't end every December 31. It starts again at the beginning.
However, at the end of each cycle, the Mayans believe in transition periods, where the old is replaced with the new. They were very concerned with the spiritual properties of every object; every item in their houses, every building in each city, and every city in relation to one another had to be placed just so. During transition periods, they would even tear down old buildings, only to replace them with new buildings with the exact same size and function, to preserve spiritual essence. Predicted for the 2012 transitional period is a time of major change for humanity. Something that isn't entirely insignificant. It might not even be sudden. It could possibly take years for any change to become noticeable.
Now, I'm about to get pretty sciency, so try not to fall asleep. You've probably all heard about the solar maximum that will occur in 2012, right? (A solar maximum being a peak in the sun's sunspot activity, which results in lots of flares) This is just one in a series of coincidences and alignments surrounding 2012. The planets will be in a particular alignment, and the Earth, sun, and the center of the Milky Way itself are supposed to align. Now you may be thinking, "Doesn't that happen every year?" Well, not exactly. You see, there's this thing called the "galactic plane." It's an imaginary plane on which most of the matter in the galaxy resides. However, most bodies, including our sun, are either slightly above or slightly below the galactic plane at any given time, not precisely on it. We are currently above the galactic plane, but only just. In December 2012, the Earth and sun will cross the plane and go to the underside of the galaxy. This may have no effect at all, but more likely it will have complications for us.
Going slightly off subject, we are now going to discuss polar swaps, where the north magnetic pole becomes the south magnetic pole, and vice versa. Scientists know this happens by studying rocks on the sea floor. As new floor is created and pushed away from the mid-ocean ridges, they contain iron atoms. As the rock is solidifying, the iron atoms align themselves with the magnetic poles. Now, every 20,000 years or so, the iron atoms switch polarity, meaning the magnetic poles of the planet must have swapped. This is important because, oddly enough, the solar system crosses the galactic plane every 20,000 years or so. This is probably more than coincidence. So this is how it all comes together.
Here's what I believe will happen in December 2012. We're going to cross the galactic plane, and the Earth's magnetic poles will begin to switch places. This will, as a result, temporarily weaken the magnetic field protecting the Earth from the highly-charged solar wind. Combine that with the solar maximum that's occurring at the same time, and that spells trouble. At this point, I'm not sure how severe the damage will be. At the least, we'll have to fiddle with our compasses and fix a few satellites. At the most, the highly charged particles coming in from the sun will the electronic infrastructure of society, and we'll have to start from scratch all over again. There will most likely be massive amounts of chaos, rioting, and the general collapse of society as we know it. Things could end up very like they are in the Fallout games, except with less radiation.
All I know is 12/21/12 will be an interesting day, to say the least, even if nothing happens. Nobody knows what will, no matter how much they claim to. It's all in God's hands.