French novelist Jules Verne delighted 19th-century readers with the tantalizing notion that a journey to the center of the Earth was actually plausible.

Since then, scientists have long acknowledged that Verne's literary journey was only science fiction. The extreme temperatures of the Earth's interior—around 10,000 degrees Fahrenheit (5,537 Celsius) at the core—and the accompanying crushing pressure, which is millions of times more than at the surface, prevent people from venturing down very far.

Still, there are a few things known about the Earth's interior. For example, geophysicists discovered that the core consists of a solid sphere of iron and nickel that comprises 20% of the Earth's radius, surrounded by a shell of molten iron and nickel that spans an additional 15% of Earth's radius.

That, and the rest of our knowledge about our world's interior, was learned indirectly—either by studying Earth's magnetic field or the way earthquake waves bounce off different layers below the Earth's surface.

But indirect discovery has its limitations. How can scientists find out more about our planet's deep interior?