In Jules Verne’s 1864 classic of  science fiction, A Journey to the Center of the Earth, an intrepid German scientist named Liedenbrock purchases an ancient Icelandic manuscript and decodes its script to discover an incredible secret. The manuscript explains that the crater of a certain Icelandic volcano is the entrance to an underground passageway that leads to the Earth’s core. Liedenbrock and his nephew Axel follow the directions, and descend into a underground world-within-a-world, full of petrified trees, mastodons, and a 12-foot-tall apeman that they theorize may be part of a vanished proto-civilization. But the subterrannean environment’s most bizarre feature was a forest of giant mushrooms.

I knew that the LYCOPODON GIGANTEUM attains, according to Bulliard, a circumference of eight or nine feet, but here were pale mushrooms, thirty to forty feet high, and crowned iwth a cap of equal diameter. There they stood in thousands. No light could penetrate between their huge cones, and complete darkness reigned beneath those giants; they formed settlements of domes placed in close array like the round, thatched roofs of a central African city.

Verne’s vision of the Earth’s interior, was, of course, more fiction than science, though his vision of underground proto-humans was inspired in part by Charles Lyell’s 1863 book Geological Evidences of the Antiquity of Man, which cited archaeological and geological evidence  that humans had been around for far longer than the roughly 7,000 years that Biblical literalists dictated.

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Today, we know that there are no subterannean building-sized mushrooms or herds of ancient creatures lurking beneath the Earth’s surface. But don’t feel too superior to our Victorian visionary. Our knowledge of the Earth’s inner structure is still fairly rudimentary, in part because traveling very far down into the Earth is pretty much impossible for humans, due to the fantastic pressures and heat to which reverse-astronauts would be exposed.

Verne would be disappointed to learn that the deepest passageway down into the Earth was the Kola borehole, dug by the Soviets in the early 1960s in the Pachenga mining region.  The borehole reached a depth of more than 40,000 feet, more than seven miles. But that effort only got slightly less than a third of the way through the Earth’s outer layer, the crust, which is as much as 25 miles deep under the continents. 

What we do know about the Earth’s core paints a picture nearly as astonishing as the inner world envisioned by Verne. Beneath the crust is an 1,800-mile-thick layer called the mantle, which is composed of rock that is semi-solid, but hot enough to flow.

Beneath the mantle lurks the Earth’s core, which actually has two parts. The 1,400-mile-thick outer core is composed of liquid iron, suphur and nickel, heated to a temperature of about 7,200 to 9,000 degrees F. Inside that is the inner core, which is composed of the same metals. The inner core is even  hotter than the outer core–between 9,000 and 13,000 degrees F–but it is under such great pressure that it cannot melt.

Here’s a basic primer on the Earth’s inner structure, from the U.S. Geological Survey.

Though scientists who belong to the international organization called SEDI–Study of the Earth’s Deep Interior–can’t actually venture very far into the Earth, they have been able to gain insights by studying seismic waves generated by earthquakes, which pass through the Earth’s core and come out on the other side of the planet. Study of seismic data, for example has revealed that the Earth’s core actually rotates faster than the planet’s surface.

Scientists also have studied fluctuation’s in the Earth’s magnetic field, which is generated by flow of molten metals in the outer core. Interestingly, they’ve determined that the flow of liquid iron in the Earth’s outer core oscillates, in waves that last for decades. Oddly, these oscillations actually correspond to tiny variations of a few miliseconds in the length of the planet’s days. 

In 2008, University of Illinois geologists used seismic wave data to verify that the Earth has an inner core, and even were able to construct a 3-D model that shows the texturing of iron crystals there.

The two-hour special X-Ray Earth premieres Sunday at 8P et/pt.

Video Preview: “See-Thru Planet” Researchers are creating 3-D models of what the Earth looks like underneath the surface.