Also known as pole shifting, it is a theory of geology propounded by the late Professor Charles H. Hapgood of Keene College in New Hampshire in his 1956 book Earth's Shifting Crust: A Key to Some Basic Problems of Earth Science. The theory basically states that it is possible for the whole earth's crust, the lithosphere, to move over the asthenosphere in one whole piece, like the skin of an orange would slide over the inside of the fruit if it were loose. It is not to be confused with plate tectonics, the gradual motion of massive plates that make up the earth's crust where the plates shift and collide against each other (the theories are however, not mutually exclusive). This is also distinct from the known shifting of the earth's magnetic poles, but refers to an actual shift of the geographic poles. Hapgood propounded this theory as an explanation for the rapid global climate change associated with the beginning and end of the ice ages, as well as an explanation for many anomalous ancient maps such as the Admiral Piri Reis Map, which contain accurate geographical information about Antarctica and South America, which were supposedly unknown at the time they were made.
If such a displacement of the earth's crust were to occur today, it would be a catastrophe of terrifying proportions. There would inevitably be massive volcanism (think 10,000 Krakatoas going off all at once) and tremendous earthquakes of unimaginable magnitude (Richter scale 12 and higher would not be unusual) all over the world. The volcanism would release huge black clouds of dust that would block out sunlight for many years (similar to a nuclear winter) and produce a temporary cooling of the world. At the same time, the volcanoes would unleash vast quantities of carbon dioxide into the atmosphere, which would later contribute to global warming once the dust of the eruptions settled. Worst of all, the former poles would wind up in temperate latitudes, remorselessly melting and flooding the whole world. At the same time, areas that were once in temperate or even tropical regions of the world could suddenly be found in polar latitudes. The sudden climate change that would result would likely bring mass extinctions all over the world. If it happened today, it would quite likely destroy all of modern civilization as we know it very quickly.
The last such displacement is believed to have occurred around 11,000 BC, at the end of the Pleistocene period, ending the ice age and bringing the beginning of the present interglacial. Hapgood theorizes that the former north pole had been located somewhere in the Hudson Bay area (60 degrees north, 73 degrees west) around 17,000 BC, and moved to its present position at 11,000 BC or so, ending the ice age. Other commentators say that such an event may well be the source for all the legends of a Great Flood or Deluge known by many widely scattered cultures all over the world. It could also explain the many strange fossilized remains that have been found deep within the Arctic Circle (and in Antarctica as well) such as tall trees and anomalous extinct animals that have no place in a glacial setting, which points to rapid and dramatic climate change.
Albert Einstein gave a ringing endorsement of this theory when he wrote the foreword to Hapgood's book. He summarized Hapgood's findings for the possible causes of these displacements of the crust:
In a polar region there is continual deposition of ice, which is not symmetrically distributed about the pole. The earth's rotation acts on these unsymmetrically deposited masses, and produces centrifugal momentum that is transmited to the rigid crust of the earth. The constantly increasing centrifugal momentum produced in this way will, when it has reached a certain point, produce a movement of the earth's crust over the rest of the earth's body.
In other words, increasing masses of ice over the poles eventually destablize the earth's rotation, causing much or all of the earth's crust to slip over its interior. Other commentators have proposed that gravitational influences within the solar system (such as might be caused by planetary alignments) also contribute to the effect. There is, however, insufficient evidence that such mechanisms can actually cause such a shift in the poles, and the topic is still very much open to debate.
Sources
Graham Hancock. Fingerprints of the Gods: A Quest for the Beginning and the End.
Pole Shift forum at http://www.poleshift.org/