Pour a few million tons of molten iron into a modest crack in the planet's surface, and the seething blob will burrow some 3,000 kilometers down to the outer core in a matter of weeks. Plant a grapefruit-size probe inside the sinking metal, and you have a sensational new way to explore the earth's inner workings.

At least that's how David J. Stevenson, a planetary scientist at the California Institute of Technology, envisions it. Some of Stevenson's colleagues have laughed out loud at his musings; others have called them "goofy." But at least a few geophysicists admit that the idea is promising, even feasible.

This article was originally published with the title Deep Thoughts.

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