Just when you thought every end-of-the-world scenario had been considered by scientists and sensationalized by Hollywood, here comes something new.

Even if we survive asteroid impacts, space germs, genetically engineered bio-weapons, nuclear war, and cloned velociraptors - now we have to worry about a possible disruption in the Earth's core that could lead to a catastrophic collapse of our precious magnetic field?

That's the premise of a new movie, The Core, that is to be released at the end of the month.

The magnetic field normally deflects electrically charged particles streaming out of the sun. In the movie, when the field fails, calamity follows: People with pacemakers drop in the streets, dead birds rain on cities, lightning bolts dance over Rio, and marauding electric currents fry the Golden Gate Bridge.

In the real world, most experts say, any changes we would likely experience if the magnetic field were disrupted would be more annoying than apocalyptic. But one researcher, albeit one in a fringe position, believes this latest doomsday scenario is not only possible but inevitable.

Scientists agree that the Earth's core doesn't just sit there - it's like a critical internal organ for our planet. It seethes and churns, creating electrical currents that generate the planet's magnetic field.

Scientists don't know exactly what would happen if the core stopped working and that field collapsed, beyond the fact that compass needles would go spinning. We might get an extreme version of events that happen during large solar flares - brilliant auroras would show up in southern latitudes; electric currents would run through transformers, creating power outages; and communication satellites would be damaged or destroyed.

It's possible that electric currents would run through metal bridges and weaken them until they collapsed. And since some birds do appear to use the field as a navigational aid, they might start flying north for the winter instead of south.

Over time, with no magnetic field at all, the excess radiation bombarding the planet could start to kill humans and other living things, slowly turning Earth into a dead planet, like Mars.

Beyond a few brief stops and starts, the magnetic field has been working pretty well for the last 4.5 billion years. But Marvin Herndon thinks the party's almost over.

Herndon is a geophysicist in San Diego, who for years has championed the notion that the Earth's core is powered by a 5-mile-diameter ball of uranium.

The uranium acts as a nuclear reactor, and it's about to run out of fuel, Herndon says. When that finally happens, Earth will lose its magnetic shield and start getting battered with charged particles.

That could occur any time between tomorrow and a billion years from now.

It's not surprising that scientists would disagree over what's going on in the planet's core. In many ways, the center of Earth is even more inaccessible than the nucleus of the atom or the outer reaches of the visible universe. With 3,952 miles of rock between it and us, it's hard to study except with clever indirect methods.

Scientists depend on meteorites to get a better idea of the composition of the material from which Earth formed. And they study deep seismic waves to map out the solid and liquid regions down there.

Such techniques have led to a standard notion of the core that describes a semi-crystalline iron inner core 1,440 miles across, surrounded by 1,300 miles of molten iron and nickel. The slow cooling of the core stirs up convective currents that drive a flow of electrons around the core's equator. This electric current turns the planet into a huge electromagnet.

Most scientists say we're in no danger of seeing this process break down. "There's no known thing that would cause that," said David Stevenson, a geophysicist at the California Institute of Technology.

However, some small change in the core could disrupt the field. The liquid core rotates as if it's a solid, he said, but it's not impossible that it could start moving in some other way.

The magnetic field does occasionally collapse and switch directions, the north magnetic pole exchanging places with the south. That happens every 200,000 years or so, leaving a record in the orientation of iron grains embedded in rocks that were molten during these events.

During these "reversals", Stevenson said, the field diminishes to about one-tenth its original strength for a few centuries. That's apparently enough to keep life going, since there's no evidence that animals or plants are going extinct as a result.

But to Herndon, the movie's disaster scenario is not all that far-fetched.

"The movie depicts clearly what will be the consequence and the peril to all humanity when the nuclear reactor dies," he said.

The movie producers did not use his nuclear-reactor theory as a basis for their plot, though they did keep on hand a copy of a Discover magazine article based on his work.

Under the current theory of the Earth's composition, with a mostly iron core, uranium would not concentrate at the center but would be locked up on rocks closer to the surface.

But Herndon said that picture is wrong - the center is not iron and nickel, but silicon and nickel.

With silicon instead of iron in the core, Herndon said, enough uranium gets concentrated to start a chain reaction and produce nuclear energy all by itself. Natural nuclear fission has occurred in a uranium deposit in West Africa, which apparently generated heat until it burned out two billion years ago.

Herndon argues that Jupiter and Saturn also have nuclear reactor cores, and this explains a recent observation that Jupiter generates twice as much heat as it absorbs from the sun. "It didn't make sense that there could still be heat coming out of Jupiter after 4.5 billion years," he said, unless it had its own source.

His latest work, which was published last week in the Proceedings of the National Academy of Sciences, relied on the computers at Oak Ridge National Laboratory in Tennessee, which can simulate nuclear reactions and predict the products.

Stevenson and others in the field say the main problem with Herndon's idea is that he hasn't devised tests that show it's more plausible than the existing picture with no uranium reactor.

"There's a chance he's right, but it's not demanded by the evidence," Stevenson said. He, like most other geophysicists, find the standard picture more plausible.

If Herndon is right and there is a reactor, it's near the end of its life. Stevenson said that when reactors are close to running out of fuel, they start producing more of a rare variety of helium - helium-3. The high ratio of helium-3 to more common helium-4 measured in volcanic rocks, he said, indicates that the end is nigh, give or take a billion years.

In the movie, scientists try to fix the problem by sending a team of "geonauts" down to the center of the Earth to straighten things out at the core.

That, Herndon said, probably would not work in real life. But people could take steps on the surface to prevent the kinds of calamities seen in the film.

"I don't look at this necessarily as a death sentence," he said. "People can find ways to survive."