The Mayans calender is based on what they call the dark plains but known to us today as a galactic alignment. This is what the dooms day scenario of dec. 21 2012 is based on and began. But it is true that we will pass through this plain around the time according to scientists today. This galactic alignment is a direct path of the milky-ways center black hole and its gravitational pull. Our galaxy passes through this plan from top to bottom or vs\vr. about every 26,000 thousand years in what is called cylical movement. For the last 26,000 years we have been above the galactic plan and are about to move to the lower section.So what effect will this have on earth? well the Mayans claim it will mark the end of this life cycle and the beginning of a new one. Scientists are not sure exactly what will happen but here are some theories of the possible effects.
1. Planetary Polar shift: North becomes south. Not sure the effect this will have on earth.
2.Comets\Asteroids hitting the earth: One of these larger ones hits the earth it is a doomsday. Because of the extra and chaotic gravity going through our solar system it will play with the kuipler belt and send comets and asteroids in abundances into the inner solar system which raises our chances of being hit.
3.Earthquakes/volcanoes eruptions/dramatic weather: same effects they allready have but on a higher level.The extra pull on the planet may cause mother nature to go wild.
4.Sun flares: Possible sun flares on the earth could cause anywhere from a radiation poisining to the earth set to flames. The sun will all so be effected by going through the galactic plain.
These are only the top 4 theories of what might happen but there are many more.
What will happen we will have to what and see but it is best to prepare just in case something does happen.