The Mayan calendar is going to end in what we have deciphered is the year 2012 on the 21st day of the 12th month. Of corse all of the little doomsday theorists have there two-bit theories as to what the Mayans were getting at by ending said calendar. Through significant research (to include all the kookie two-bit theories) I found what I believe to be the most credible answere to the question. The calendar isn't actually ending, it's restarting.
Much as the Y2K scare, people are freaking out because this little tid-bit isn't nearly as fascinating as a potential end of our exsistance (and it's less marketable). You see, one of the more fact based and scientifically explanitory diatribes I came upon explained it as such: "The date December 21st, 2012 A.D. (18.104.22.168.0 in the Long Count), represents an extremely close conjunction of the Winter Solstice Sun with the crossing point of the Galactic Equator (Equator of the Milky Way) and the Ecliptic (path of the Sun), what that ancient Maya recognized as the Sacred Tree. This is an event that has been coming to resonance very slowly over thousands and thousands of years."
Think of it as kind of like a new year, except on a much larger scale. "The Tzolkin is a 260-day calendar based around the period of human gestation. It is composed of 20 day-signs, each of which has 13 variations, and was (and still is) used to determine character traits and time harmonics, in a similar way to Western astrology. The Maya also used a 365-day calendar called the Haab, and a Venus calendar, plus others. They measured long time periods by means of a Long Count, in which one 360-day year (a "Tun"), consists of 18 x 20-day "months" ("Uinals"). Twenty of these Tuns is a Katun; 20 Katuns is a Baktun (nearly 400 years); and 13 Baktuns adds up to a "Great Cycle" of 1,872,000 days, ( 5200 Tuns, or about 5125 years)."
Wooo! Furthermore, this same article describes sunspot cycles changing and altering the magnetic field generated by the sun. The result being the earth reacting in a way that some might view as less than positive (earthquakes, floods and such). Which is probably where all these dingleberries get the hair-brained idea that the world is going to end. Still, no comet, no 'planet x', no puffed up science fiction-esqe armageddon. The article sites "Generally speaking, scientifically accepted records of sunspot activity do seem to be heading for a climax in the near future."
I digress, and leave you with something that will probably turn 90% of you against me; with a lack of substantial evidence in the cause of global warming... How about that sun's magnetic field shifting thing?
If you're curious about the whole thing, clicky-clicky; www.greatdreams.com/2012.htm