Peek into the earth

Some of the greatest mysteries of the world lie right beneath your feet. Think about it: No one has really explored inside the Earth.

Oh, some deep holes have been dug. But the deepest is only about 7-1/2 miles. That's not much, considering it's about 4,000 miles to the center of the Earth.

We've landed on the moon 240,000 miles away. Why have we only scratched the surface of the Earth? Because it's too difficult, too hot, and too expensive. But that doesn't mean people don't wonder what's down there.

In the 16th century, English astronomer Edmund Halley (of Halley's Comet fame) theorized that the Earth was hollow and had a luminous core at least partly filled with gas.

His vision inspired some fascinating fictional accounts. The most famous, "Journey to the Center of the Earth," was published by Jules Verne in 1864. The book chronicles the adventures of a geologist and his nephew as they descend into a volcanic crater. They encounter prehistoric beasts, spewing lava – even the lost city of Atlantis.

Even into the 1900s, there were some pretty kooky theories about the Earth's interior. One envisioned a place where billions of people might live. Another anticipated a space with a small sun, which could be reached through openings at the Earth's poles. (The explorer, Adm. Richard Byrd, disproved this when he flew over both poles in the 1920s and found no large holes.)

Today scientists largely agree on the basic structure of the Earth. It does not include alien hideouts or underworlds of any kind. Some diagrams compare the layers of the Earth to an egg (a round egg, mind you). The yolk represents the core, the egg white the mantle (a semisolid layer of rock). The shell of the egg is the crust, the thin brittle "skin" on which we live.

That's the theory, anyhow. It's based on scientists' observations of earthquakes and volcanoes. It's also based on experiments using small amounts of material and subjecting them to high heat and pressure. That's the only way researchers can get an idea of the effect of conditions deep underground.

Eric Muller, who works for the Exploratorium, a science museum in San Francisco, says the inner core may be as hot as the surface of the sun – about 6,000 degrees Celsius (11,000 degrees F.). The inner core is also under extreme pressure, an estimated 3.6 million times as much pressure as on the surface of the Earth. Because of this pressure, the core of the Earth is thought to be a solid metal ball surrounded by a slightly cooler molten outer core.

Scientists theorize that after the Earth formed billions of years ago it began to cool slowly. Denser materials – iron and nickel, common to our Solar System – sank into the core. Lighter elements floated near the top.

If the Earth were an apple, the crust would be as thin as the peel. So far, no one has even drilled through the crust. The most ambitious attempt to do so began in 1958 and ended in 1966. "Project Mohole" was to be Earth sciences' answer to the space program.

Project Mohole's goal was to reach the point where the crust and upper mantle meet. This region is called the Mohorovicic Discontinuity, after Croatian geologist Andrija Mohorovicic, the first to suggest its existence.

Because the crust is thinnest on the ocean floor, Project Mohole began drilling off the coast of Mexico, 11,700 feet underwater. The drill reached 601 feet below the seafloor. It stopped short of the "Moho." Congress ended the project.

Deeper holes have been drilled on land, including oil wells of 20,000 feet. A research borehole on the Kola Peninsula in northeastern Russia was twice that depth.

Miners in South Africa routinely go deeper into Earth than any other humans. The deepest gold mine there is 2-1/8 miles below the surface. Temperatures can be unpleasant at that depth, reaching 120 degrees F.

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