There have been countless theories throughout time about how the world will end and how - or if -life will cease to exist. At the turn of the 21st century, conspiracy theorists claimed that the Y2K bug was only a small part of the impending devastation: The new century would bring about total destruction, and no one would survive. Others believe that Earth is slated for another ice age, which will kill off all living things. And according to astronomers, billions of years from now, the sun will become a red giant, expanding to a size larger than the Earth's orbit and consuming Earth in the process. Even if the planet somehow survives, the sun will eventually shrink, becoming a white dwarf and gradually cooling off until it can no longer warm anything in the solar system.

In 2006, Mel Gibson released a movie about the Mayan civilization. "Apocalypto" follows one man's journey from slavery back to his family. During the course of the movie, a young woman prophesizes that a man will bring an end to the Mayans and wipe out their civilization. But in the real world, some speculators don't believe a man will be the end of the Mayans -- instead, they think a celestial event will be the cause. The Mayan calendar even gives us a potential date for the theoretical downfall: Dec. 21, 2012.

Many doomsayers hang onto the idea that this ancient calendar is a ticking time bomb signaling our fast-approaching demise during the 2012 winter solstice. While scholars pay little heed to these fervent forecasts, they're still interested in the calendar. For example, there is some lingering disagreement over which day marks the exact beginning of the Mayan calendar. One commonly referenced date is Aug. 11, 3114 B.C., although other researchers pinpoint dates such as Aug. 13, 3114 B.C. (which would make Dec. 23 the big day.)

Complicated Calendars

The Mayans have a complex system of calendars, and each calendar has a different purpose. Some of the more commonly known Mayan calendars are:

The Tzolk'in Calendar: Used primarily in crop rotation, this calendar allows one 260-day period to ready the land and one 260-day period to grow and harvest the corn. It was also considered a sacred calendar by the Mayans and determined when their rituals occurred. The Tzolk'in uses 20 days signs coupled with 13 number signs to produce the 260 uniquely identified days.

The Haab Calendar: This calendar lasts 360 days and breaks down into 18 20-day months, with a five-day waiting period at the end that was considered extremely unlucky called the wayeb (also sometimes spelled uayeb). Similar to the Gregorian calendar we use today, the Haab calendar follows the cycle of the sun.

The Calendar Round: The Calendar Round gave the Mayans a way to record history in longer increments. By combining the Tzolk'in and Haab calendars, they had 18,980 uniquely designated days, or in other words, 52 years.

Fifty-two years was longer than the Mayans' average life span at the time of the Calendar Round's creation. However, Mayan historians wanted to create a calendar that could be used to record history for centuries. This led to the Long Count calendar. The Long Count multiplies up from the other calendar systems and incorporates an era called the Great Cycle, which lasts approximately 5,125 years [source: Malmström]. The idea that the world is on its way to an end comes from the Long Count.

The Long Count to the End
Use of the Long Count calendar dwindled after the Mayan Classic period and was apparently nonexistent by the time the Spanish arrived. Since no extant records exist for researchers to draw direct correlations, many have attempted to extrapolate from the progression of the other Mayan calendar records using logical deductions. Through this work, it's been determined that the beginning of the current Great Cycle likely fell on Aug. 11, 3114 B.C., making the end of the Great Cycle land on -- you guessed it -- Dec. 21, 2012. (Or potentially Aug. 13 and Dec. 23, depending on how scholars calculate it.) Apocalypse aficionados and others anticipating a grand climactic change seem to be especially set on designating Dec. 21 the big day. After all, it does sound decidedly sexier to say the world might meet disaster on the winter solstice as opposed to a rather insignificant date two days later.

Regardless of this debate, the Mayan people don't actually believe the world is going to perish at the end of this cycle. In fact, they believe it's a time of great celebration and luck when the planet lasts through a full Great Cycle. And after all, it seems we've made it safely through past Great Cycles (it's debated how many cycles there have been) and the world is still turning. Also, to help put this in perspective, chances are good you own a calendar that stops short on Dec. 31 of every year -- does that strike you as a herald of disaster or just the sign of a new beginning?

Proponents of an impending December doomsday, however, believe the world will end and all living things on Earth will die. Other people believe it will signify a time of great spiritual awakening or a massive shift in global consciousness. But is this monumental turn of a vast page in the Mayan Long Count calendar really going to end the world or radically alter worldwide perspectives? Probably not. Most scholars see this theory as an example of extremists using misinformation and nonsense to cash in on the fears of others or tailor an overdue apocalyptic event to fit their own pre-existing belief system [sources: NASA, Tulane University, New York Times].

So what will happen on Dec. 21, 2012? It's likely that the day will pass with no major events at all. People may not even realize it's the projected doomsday, although that's unlikely considering how much press it will probably garner. We'll just have to wait and see what happens -- and hopefully still be here to update this article on Dec. 22, 2012.­­