Through centuries long observation of the skies and carefully calculated mathematical algorithm, the Mayans have pinned December 21, 2012 as the end of the current Long Count Cycle.  The current cycle is the fourth of five cycles that make up the Grand Cycle.  Hence, once the fourth cycle ends, the fifth cycle begins. 

December 21, 2012 is NOT the end of the Mayan calendar as many would put it.  Just the end of one of it's cycles.

At the end of the current cycle, a few things are predicted.  Of course, they're all theories, the majority of which are doomsday scenarios.  The ancient Mayans actually see this as a "cleaning of the slate" and starting new for the next cycle.  Perhaps it's no coincidence that December 21, 2012 is a solstice day, an event observed by many cultures, past and present, as the ending of a season and the beginning of the next season.

Let's start with the fun stuff -- Armageddon. It's not clear exactly what's gonna happen on the day of December 21, 2012, but there are some interesting predictions and they're not necessarily exclusive of each other. Meaning that one catastrophe may cause others to occur, such as when earthquakes cause tsunamis:

• magnetic pole reversal
• massive solar flare
• comet-Earth collision
• super volcanoes
  

There are also non-destructive theories on what's about to happen based on Mayan beliefs and practices. But they are more abstract. According to Mayan beliefs, Quetzalcoatal, the supreme Mayan god, will return to establish the next world. During this time, there will be a rising human consciousness. Meaning that we will be more synchronious and attune with each other and to nature, specifically, to the natural cycles. We will also have more energy awareness.

What do these mean? Well, to me and if you're a fan of Stargate SG-1, then this theory sounds a lot like "ascension". It's when the Ancients, an advanced fictional race of humans, left this universe for the next and became metaphysical beings.

What's north is south and what's south is north. Up is now down and vice-versa. Not a hard concept to understand. Pole reversal is when the geomagnetic orientation of the North Pole switches to that of the South Pole and vice versa. When that happens, the needle on a compass will point down rather than up.

Apparently, magnetic pole reversals naturally occur in earth's long timeline, on the average of 250,000 years. And a reversal has not occured in over 700,000 years. However, some scientists believe that we're in the middle of such a reversal right now.

Pole Reversal Effects: So what if it happens?

1) Navigational instruments -- Transportation relying on compasses will be heavily affected by a polar reversal. So airplanes and boats will have trouble manuevering and may have to be shutdown until a solution is found. This will effectively stop commerce that we take for granted.

2) Migratory animals -- Ever wonder how birds, sea turtles, fish, and other migratory animals come to navigate around the world? We, humans, get by using Google maps nowadays or pay someone to get us there, e.g. airlines and cruise ships. Though scientists are not clear yet on exactly how migratory animals "instinctively" find their way to their seasonal destination, mostly to mate and have babies, some scientists theorize that these animals use the Earth's magnetic field. If that's true and a polar reversal occurs, then migratory animals who rely on the magnetic field to travel will be disoriented. Thus, we'll be seeing birds and turtles in places where we don't normally see them. Though not necessarily bad, directional disorientation has been theorized as a reason why we find healthy whales dying on beaches.

Ever see the 2009 movie, Knowing, starring Nicolas Cage and Rose Byrne? In Knowing, the world ends when a wave of intense solar flares occurs and scorches everything on Earth. It's a doomsday movie and only a handful of children have been selected to start over in another planet or universe. Perhaps the beginning of the fifth cycle from the Mayan Long Count Calendar ...

In 2012, NASA is predicting a solar maximum that we haven't seen since 1958. Solar maxiums on the surface of the Sun are what drive solar flares to Earth and other planets in the solar system. They're a regularly occuring natural phenomenon.

2012 Solar Maximum - Highest Intensity in 50 years

What's important is that the 2012 solar maximum intensity (Examiner.com) is expected to be the highest in 50 years. The solar maximum in 1958, though very intense, did not affect us as much back then because our technology did not rely so much on electricity and magnetic signals.

Unlike today ... wireless Internet and cell phones anyone?

In fact, it doesn't have to be wireless to be affected. Modern day electric grids have risen exponentially in the past 50 years to power-up industrialized cities. But they have not been insulated properly from intense solar flares (see Examiner.com article).

Anything from banking, online shopping, satellites, telecom, etc. will be disrupted. For how long? Who knows. Imagine if you're getting direct deposit or selling online. What if airports are shutdown and no airplane can fly.

I suppose it's better to hope for the disruption of commerce because that means the world will continue. But go see Knowing. It's actually a pretty good movie. Also read this article on UniverseToday.com, titled "2012: No Killer Solar Flare."

Not so much a Mayan prophecy, but still a popular 2012 prediction is a comet hitting the Earth to doom us all. There's an entire website around this subject: 2012-comet.com. But let's make this simple and lump planets, asteroids, and other large heavenly bodies hitting the Earth to end all life in this particular section.

Science has backed this possibility by a popular theory attributing the demise of the dinosaurs from an asteroid hitting the Earth. Also, as recently as 1994, scientists have observed Jupiter being hit by a comet in a spectacular display of cosmic power. So planets being hit by extraterrestial objects happen. And they happen all the time.

The theory of the world ending through a comet-Earth collision actually comes from the Bible codes according to 2012-comet.com. Wormwood is how the Bible refers to he life-ending comet.

Often described as 1000 times more powerful than a regular volcano (think Mount St. Helen but a thousand times stronger), super volcanoes are naturally occuring phenomenons that can destroy life as we know it.  They are formed differently from a regular volcano in that super volcanoes are formed as a depression on the landscape called a caldera.  While normal volcanoes bulge outward forming mountains or hill regions.

The best known super volcano today is in Yellowstone Park in Wyoming, a very popular recreational park known for Old Faithful Geyser.  Around 2-3 million visitors come to Yellowstone annually for camping, hiking, skiing, and animal watching.  I've seen documentaries indicating that Yellowstone is the best place in the US to safely observe wolves in the wild.

But underneath the flora and fauna is an immense red hot reservoir of magma around 30 miles long, 20 miles wide, and 6 miles deep.  A Yellowstone eruption will implicate the world.  The immediate states adjacent to it will be devasted by molten lava, magma, and earthquakes.  The rest of the world will suffer from ash cloud that could block the sun for years.  It will be like the nuclear winter that was often talked about if nuclear war was to happen.

Many 2012 theorists believe that if Yellowstone was to erupt, it will do so because of the solar flare that's expected on December 21, 2012.  Yellowstone have been building up to explode throughout the years and the added energy of an intense solar flareonto the Earth will push the supervolcano over the tipping point.