It's not just the Mayan calander in fairness. I actually did a bit of research into the 2012 thing a couple of years ago, and this was my summary of info I posted on my fb... (though obviously, the info could be completely out of date and have been utterly disproven by now 

)
Nostradamus
Nostradamus predicts the passing of a 
comet causing turmoil and misery.
He  also says a solar eclipse more obscure and dark than any since the  creation of the world will hit us cause flooding, earthquakes, a halt to  the earths gravitational movement causing perpetual darkness and 73  years and 7 months worth of recovery time. 
And this will be  preceded by a solar eclipse more obscure and more dark [and mysterious],  than any since the creation of the world except for the death and  passion of Jesus Christ, from that time ill now, and there shall be in  the month of October some great movement and transference [of the globe]  and it will be such that one will think that the Earth has lost its  natural [gravitational] movement, and that it will be plunged into the  abyss of perpetual darkness; there will be initial omens in the spring,  and extraordinary changes in rapid succession thereafter, reversals of  kingdoms and mighty earthquakes, with the increase of the new Babylon,  the miserable daughter, augmented by the abomination of the first  Holocaust, and it will last no less than 73 years and 7 months…” 
The Bible Code
Bible codes describe a comet causing a polar shift in 2012. 
2012 – comet – large - stone-like / stony – object - sped 
Tongue-like – Sun - whole earth / earth annihilated - smitten / stricken - smitten 
Planetary / wanderer – fragmentary – sixty – mile - blunderbuss / scatter-gun 
Crater – Canada – ultimate – terrifying – appalling – lethal 
Modifier - mantle - axis - tilting - tipping – Speeded 
Seismic - eradication - It will be crumbled / I will tear to pieces 
Cosmic alignment
21st  12th 2012: Sun lines up directly with the centre of the milky way. No  one knows what might happen. Most astronomers are now convinced there is  a black hole the size of millions of our suns at the centre of the  milky way. No one knows what this could mean in terms of energy and  time/space. 
Solar flares
Every 11 years there is a  solar pole shift, flipping the magnetic field of the sun 180 degrees.  This could send out a larger solar flare called a 
coronal mass ejection.
On  dec 21 2012 saturn and jupiter line up, effecting the suns regular pole  shift with it's own gravitational effects, increasing the chances of a  devestating solar flare. 
The chances of this are said to also be increased by the passing of 
planet X.
		
		
	
	
A Coronal Mass Ejection
Polar shifting
The  earths magnetic north pole and south pole are in allignment. The sun's  pole switching, combined with the saturn and jupiter influence may cause  the earth's poles to switch as well, which would cause massive  devestation (earthquakes, volcanic eruptions. After this north will be  south and the sun will rise in the west.
We could also experience a polar wandering, in which the tropics would become the north pole. The Us could end up in ice.
Planet X is also a possible cause of a polar shift.
Planet X/Nibiru
Nibiru, according to the ancient sumarians, comes into our solar system every 3,600 years. 
It is said that Nibiru's strong gravitational effect will cause earth quakes, super volcanic activity and 200+MPH winds.
When  it is at it's closest too us, a quick pole shift will occur, causing  the oceans to rush over the earth, followed by the world being  encompassed with 
volcanic ash. 
Nibiru's Orbit
Underground volcanic eruption
2012 is the end of a 74,000 year cycle of a super underwater volcano.