In 1970 Russian geologists started drilling into the Kola Peninsula, near Finland, hoping to learn more about Earth’s enigmatic insides. After 22 years of digging, work had to stop when the crust turned gooey under the drill bit; at 356 degrees Fahrenheit, the underground rock was much hotter than expected at that depth. The result of the scientists’ grand effort: a tunnel as wide as a cantaloupe extending all of 7.6 miles down.
The Kola borehole is by far the deepest one ever dug, yet it reaches a mere 0.2 percent of the way to the core. The rest of Earth’s interior remains as frustratingly out of reach as it was three centuries ago, when astronomer Edmond Halley suggested that our planet was hollow and filled with life. His ideas seem laughable today, but the truth is, when it comes to the inner Earth, no one knows anything for sure. Might a massive crystal sit at the center? What about a natural nuclear reactor? Are we so sure that the textbook diagram of the Earth sliced open, with nested layers of yellow, orange, and red, reflects reality?
The questions are so compelling that they inspired one geophysicist to draw up blueprints for a journey to the center of Earth. Nobody is doing it just yet; it would require cracking open the ground and pouring in thousands of tons of liquid metal. But that and other far-fetched ideas may inspire the ambitious projects necessary to catch a glimpse of the core—a place just 3,950 miles below our feet and yet, in many ways, less accessible than the edge of the visible universe, 13.8 billion light-years away.
