By Dr David Whitehouse
BBC News Online science editor
It is not just the plot for a far-fetched science-fiction disaster movie. Something unexplained really is happening to the Earth's magnetic field.
Finding the shifting magnetic pole
In recent years, the field has been behaving in ways not previously seen in the admittedly short time it has been monitored.
Some researchers think it may presage a geomagnetic reversal when the north and south magnetic poles flip.
Such speculation takes place as the science-fiction movie The Core goes on release. In the film, the Earth's core stops rotating and our planet's magnetic sheath collapses.
A manned mission is despatched to the centre of the Earth to "jumpstart" the planet.
Scientists admit there are things going on way beneath our feet that they do not understand, and which could have profound consequences for life on the surface.
The Earth's magnetic field is caused by motions in the ball of molten iron that lies at the centre of our planet. Electrical currents in the outer part of the core result in the planet-wide magnetic field.
The magnetic field not only shields us from harmful cosmic rays but also funnels charged particles shed by the Sun towards the magnetic poles, where they can produce beautiful auroral displays.
Journey to The Core - science fiction
The field is also an invaluable aid to navigation. Even though today we have the pinpoint accuracy of the satellite-based Global Positioning System, many still rely on their compass needle pointing to magnetic north.
But as all who use a compass know, the position of the magnetic pole changes and there is evidence that it is changing at an increasing rate.
Working for the Canadian Government-funded Geolab, it is Larry Newitt's job is to track the wandering north magnetic pole.
Every few years he undertakes a seven hour flight from his base in Ottawa to Resolute Bay, the closest inhabited spot to the magnetic pole.
Then it is a three-and-a-half-hour flight north in a Twin-Otter aircraft which will land on ice. Today the pole is at sea and the expedition can only be done at the end of the winter when the sea is frozen.
THE WANDERING POLE
Rock samples have determined the changing position of the magnetic pole over the past 7,000 years
Placing magnetic sensors on the ice the expedition attempts to surround the magnetic pole and triangulate its correct position. But each time they go back it's moved.
"We're following it across the ice," Larry Newitt told BBC News Online. "It jumps around from day to day and year to year and we have to keep track of it."
Measurements of the magnetic pole's position in 1904 by explorer Roald Amundsen put it in roughly the same place as an earlier though less accurate measurement made in 1831 by the British explorer John Ross.
Since then it wandered slowly northward until about 30 years ago when it started behaving differently.
"There was a slow drift northward but it then started to move faster. It is now moving northward, away from Canada to Siberia, at a rate some four times faster than it used to," said Dr Newitt.
Soon, he added, expeditions to the magnetic pole would become more difficult as it moved out of range of the Twin Otter aircraft.
The reason for the wandering of the magnetic pole is twofold. One cause is from beneath our feet, the other from above our heads.
Short-term jitter is caused by the influence of the solar wind on the Earth's magnetic field high in the atmosphere. But the steady drift reflects what is going on in the Earth's core.
But something else is happening to the Earth's magnetic field: it is getting weaker.
The Core: Some science, some nonsense
David Kerridge, of the British Geological Survey, told BBC News Online: "There is strong evidence that the field is decreasing by about 5% per century."
Some researchers suggest that it could be the start of a geomagnetic reversal, when the strength of the Earth's magnetic field decreases and then returns a few thousand years later with the north and south magnetic poles reversed.
Looking back in the geological record it is clear that on average such events occur about every 250,000 years. However, it has been 750,000 years since the last reversal - so we are certainly overdue.
Magnetic measurements made on the surface suggest that a region of the Earth's core under South Africa is of a different polarity to the rest of the magnetic field in the core. It may grow and initiate a flip, or it may die down.
Whatever happens will not happen quickly. It will take thousands of years and there is no evidence that when it has happened in the past it has seriously affected life on Earth.
As for The Core, David Kerridge said it was absolute nonsense; whilst Larry Newitt told BBC News Online that he believed it "is composed of a few scientifically plausible ideas mixed with a large dosage of sheer nonsense. It should be fun."