The date also marks an end point in the Mayan long-form calendar, the source of most doomsday predictions.
There are numerous end-times possibilities espoused by adherents, most prominently a large planet called Nibiru, or Planet X, that will collide with Earth or pass close by it.
With about a month left until Dec. 21, Nibiru should be visible to the naked eye by now, and NASA reports nothing of the kind within the solar system.
Believers say NASA is covering up Nibiru's approach from behind the sun, but that explanation wouldn't account for thousands of amateur astronomers who constantly watch the sky, according to JPL senior research scientist Don Yeomans.
"Since the beginning of time there have been literally hundreds of thousands of predictions for the end of the world," Yeomans said in a JPL-produced video released in May. "And we're still here."
Other reasons put forward for the 2012 apocalypse include a planetary alignment with the Milky Way (not true, NASA says), a reversal of the magnetic poles (not imminent, harmless) and severe solar storms that could knock out electrical grids and satellites (partly true, but risk is minimal).
A meteor impact is the likeliest cause of future cataclysm, and JPL's Near-Earth Object Program watches for any coming hazards.
The 2012 prediction has nevertheless captured people's attention, including a Los Angeles-based survival group that gathered professional experts such as nurses and security officers in the past few months. The group's deadline to join was Oct. 31.
The 50-member group is anticipating more realistic scenarios, including economic collapse or a super volcano, and is preparing itself to live off the land at undisclosed locations.
"We are not certain that something will happen in December or in the near future, but we are preparing for whatever happens now or in the future with training and knowledge," group founder Andrew Brown said in an email.
More students have signed up this year for classes with survivalist Christopher Nyerges, who teaches independently and at Pasadena City College.
Nyerges suspects some of his students join because of 2012 fears, but he specifically dismisses those claims in class.
"I'm not preaching 2012, I'm teaching you stuff that is valuable every day of the week," he said.
A modern-day Mayan cultural group in Guatemala, Oxlaljuj Ajpop, issued a statement in October blasting doomsday claims and promotions, according to reports. The group is planning sacred events connected to the Dec. 21 date, which represents the end of a calendar cycle and the beginning of a new one.
Nyerges, who has studied Mayan cosmology, said most interpretations predicting the end of the world confuse religious and secular aspects of Mayan culture, which maintained separate calendar cycles.
"The bottom line is it's just bogus that the Mayans said anything about any disasters," he said.