In the movie The Core, scientists travel about two thousand miles below the earth’s surface—and reach only the outermost edge of the earth’s core. The movie offers a smidgen of science and a generous dose of science fiction.

But as this ScienCentral News video reports, it does make you wonder what the core of the earth is really like.

What lies beneath?

It’s about four thousand miles from the surface of the earth to its innermost center. So far, the deepest anyone has been able to drill is about seven and a half miles. At that depth, rock becomes so hot that standard drill bits can melt, and the drill hole threatens to collapse. (Drawing on the advice of a nanotechnologist at UCLA, The Core solves this problem with a new metal dubbed “unobtainium.”)

So what do scientists know about the interior of the earth, and how do they know it?

In The Core, an actor playing a geophysicist uses a halved peach as a model for the earth’s interior. Jeffrey Park, a geophysicist at Yale University, says the movie’s model represents a cross section of the earth quite well. “We live on the crust, a very thin layer of fairly light rock about 30 miles thick,” Park says. “Relatively speaking, the earth’s crust is actually thinner than peach skin. Most of the earth’s interior is dense, heavy rock called the mantle, represented by the flesh of a peach. The mantle is about 1,800 miles wide. The earth’s core is slightly bigger, relative to the planet, than the pit is to a peach. And while a peach pit is solid, the earth’s core is half liquid and half solid. The inner core is solid iron-nickel, while the outer core is incredibly hot, churning liquid iron and nickel—swirling liquid metal like a lava lamp—which is simulated very well in The Core.”

According to Park, all earth scientists believe that when the earth was first forming from a mix of fiery material about 4.5 billion years ago, the interior became so hot that iron melted out of solid rock. Because iron is very heavy, it sank to the middle of the new planet, so that iron and rock separated. Iron formed the core, while rock became the mantle, and the lightest rock formed the crust of the earth. In Uncle Tungsten, neurologist Oliver Sacks’s memoir of childhood, he recalls, “I was reassured when I learned that the core of earth consisted of a great ball of iron—this sounded solid, something one could depend on.” And almost every geophysicist agrees that uranium, which tends to bond chemically with rock, especially the lightest rock, became concentrated in the earth’s crust.

J. Marvin Herndon is the exception. An independent geophysicist, he holds a unique view of the core. From his studies of the gas planets Jupiter, Saturn and Neptune, and of the mineralogy of a class of rare meteorites called enstatite chondrites, Herndon hypothesizes that when melting iron sank into the earth’s core, uranium sank with it. As a result, Herndon believes that there may be a three mile-wide nuclear reactor at the innermost center of the earth. This natural reactor, he says, fuels the earth’s invisible electromagnetic field, which repels the sun’s dangerous radiation and prevents it from burning the earth.

“There is plenty of debate in the scientific community over the nature of the core,” says Jeffrey Park. “Because the inner core is the very bottom of the planet, any clue we have about its nature is of great interest. But Herndon’s theory is so far outside the mainstream that hardly anyone is paying attention to it.”

Most earth scientists support their own views of the core by pointing out that the meteorites that Herndon studies are very unusual. They cite the fact that common meteorites, which are thought to be part of a single early planet that broke apart before earth formed, are made of either rock or an iron-nickel alloy. Geochemists add that uranium tends to bond chemically with rock, especially the lightest rocks, so that it is concentrated in the earth’s crust, rather than the mantle. Park points out that uranium atoms are exceedingly rare in the iron-nickel meteorites, supporting the chemical argument for uranium’s remaining rockbound when the core first formed.

From evidence provided by waves generated by large earthquakes, most seismologists—scientists who study earthquakes—believe that the fast-moving liquid metal of the outer core generates the earth’s electromagnetic field, not a natural reactor. During a large earthquake, two kinds of seismic waves travel down through the earth and rebound off the core, providing scientists with information. The first kind is a pressure wave which, as The Core’s geophysicist demonstrates in his classroom, travels like a sound wave. The second is a shear wave, a sideways motion that Park says is “a little bit like a wave that goes around the upper deck of a baseball stadium. The interesting thing about a shear wave is that it will not travel through liquid. Shear waves will not travel through the outer core. That’s why we know it’s liquid.”

Herndon counters that simulations that he carried out with nuclear engineer Daniel F. Hollenbach at Oak Ridge National Laboratory confirm his hypothesis that a nuclear reactor is at the very center of the earth. He also points out that lava flows from Hawaii, which sits atop a large column of hot rock that may rise from as far down as the boundary where the mantle meets the outer core, contain fission products light enough to have traveled up from the innermost core.

But at present, mainstream geoscience’s focus is elsewhere. Some geophysicists are debating the details of exactly how the motion of the liquid that makes up the outer core generates the earth’s electromagnetic field. As for what’s going on in the inner core, “there are a lot of ideas, some very clever and imaginative,” says Jeffrey Park. “For example, seismologists think they see evidence that seismic waves travel faster in one direction in the inner core. That tells some scientists that the inner core is actually a single crystal, or a bunch of crystals lined up in one direction. This is very exciting, because it suggests that the solid iron of the core might actually still be moving.”

In The Core, an actor playing a prominent geophysicist says, “All of science is a best guess.” In earth science, there is some data to support a number of different views about the core—which is why geoscientists’ debates rage almost as hotly as the center of the earth itself.