The Mayan Calendar seems based on much older calendars, like that of the Olmec peoples, and maybe in turn, based on truly ancient knowledge.

So what is about to happen?

At dawn on the 21st December 2012, at the Great Pyramid near Cairo, the Sun will rise, and be aligned with the plane of our Milky-Way Galaxy, horizontal, flat along the horizon.
See Planetarium Picture

The obvious and self-evident Environmental Crisis is about to crunch, after years of protest and awareness, and human hardship, in many parts of the world.

It maybe more of a spirtual awakening, than any kind of physical calamity.

It may not be until around 2300 or 2900, that the predicted planet, Nibiru is re-discovered, as an approaching red crescent. Re-entering the solar system on its 3450 year orbit, passing through the Asteroid Belt, between Mars and Jupiter.

Reference the Anunnaki.

We have reached the turning point in our history, an awareness that our unsustainable activities can no longer continue. The destruction of the Environment, and the impending Global Warming, Climate Change Crisis which is now accelerating out of control. Because of all the Pollution, Emissions, Jet Planes, and CO2 etc we are now stuck, as it were, between a rock and a hard place, as the planet of Heaven re-approaches...

What happens 229 years from now, when the Nippur/Hebrew Calendar gets to 6000?