Every few hundred millennia, the earth undergoes a radical change that sees the planet's magnetic poles flip.
The reversal - which would cause all compasses to point "south" rather than what we currently consider north - is long overdue, and some doomsday theorists believe the switch could come on December 20 or 21, when the current cycle of the Mayan Long Count calendar ends after 5,125 years.
Some scientists are also concerned, but their worries stem largely from uncertainty about the polarity reversal rather than fears the end is nigh.
Professor Chen Gengxiong , a researcher with the Institute of Geology and Geophysics at the Chinese Academy of Sciences (CAS), noted that the last magnetic pole reversal happened about 780,000 years ago - or about two times as long as it normally takes, according to geologic records.
And concerns over the unclear timetable are magnified by the fact that the magnetic north pole has moved up by about 1,100 kilometres since the 1800s - a phenomenon that Chen said some geophysicists consider the start of the latest reversal.
It appears to be a gradual process, but a popular doomsday hypothesis posits that, if the reversal were to occur in a single day, all of humanity would be destroyed when the world stops spinning, or perhaps because the magnetic field would cease to protect people from deadly cosmic rays.
"That will not happen," Chen said. He did not, however, go as far as to say that fears over an apocalypse were total nonsense.
The heart of every popular doomsday theory was something that could not be fully explained or predicted by modern science, Chen said.
In the last 20 million years, magnetic pole reversals have been a regular occurrence, according to geophysicists who have studied ancient lava flows that show the orientation of magnetic fields. But contrary to doomsday theories, the reversal has never happened in a single day. It takes tens of thousands of years, if not longer, Chen says.
And palaeontologists say the reversal has never coincided with a major mass destruction event, such as the extinction of the dinosaurs, and has never caused the planet to stop spinning or the magnetic fields to disappear.
"People's fears about a magnetic pole reversal stem from our failure to determine the origin of the planet's magnetic field," Chen said.
After more than a century of trying, scientists are no closer to physically probing the centre of the earth. Instead, they must rely on seismic waves to deduce the physical state of the planet's core, but no one is sure how or why the magnetic field is generated.
"High school textbooks all say the earth's core is a ball of liquid metal, likely iron, and that it generates the magnetic field, but this is a wild guess. The origin of the magnetic field remains one of the biggest unsolved mysteries in science today," Chen said.
Professor Sun Xiaochun , the deputy director of CAS's Institute for the History of Natural Science and an astronomer familiar with ancient calendars, said the end of the Mayan calendar meant the beginning of a new Long Count period.
"The event will be as destructive as when we throw an old table calendar into the rubbish can at the end of the year," Sun said. "Chinese ancient calendars also had many 'circles'. If we take all of them into account, every year contains a doomsday."
Some scientists had proposed the end date of the present Long Count was actually June 6, Sun said. "The debate arose over an uncertainty about the calendar's beginning date. If one can't determine the beginning, the end would of course be debated."
Some Chinese researchers have found that certain tribes belonging to the Yi ethnic minority in the forests of southwestern China had been keeping an 18-month calendar similar to the Mayans. The Yis used the calendar to predict events rather than guide agricultural activities, as the calendar was not related to the changing of seasons.
Some people also believe that the end will come when a small planet collides with the earth.
But Professor Zhu Zi , an astronomer with the CAS Observatory in Nanjing , said that it was unlikely.
Zhu said that many countries, including the United States and China, had developed observation networks to track potentially dangerous asteroids, and nothing had been found that could wipe out earth - at least not in the foreseeable future.
But Zhu said that people still had good reasons to worry. With the current technology, an incoming asteroid can only be predicted a year or two in advance. As the path of many asteroids are affected by undetected gravitational pulls, their orbits change frequently.
"What I can say is that the impact will not happen this month. And it is very unlikely to happen next year," Zhu said. "Beyond that, we don't know."
