In 1909, Croatian seismologist Andrija Mohorovičić made a bold prediction—that Earth consists of distinct layers of rock above its core. On analyzing an earthquake that had struck earlier that year, he noticed that seismic waves below a depth of about 56 km travel a few kilometers per second faster than those above that depth. The abrupt change in speed marks what is now known as the Mohorovičić discontinuity, or Moho—the boundary between Earth’s crust and upper mantle, where a fundamental change in the rocks’ composition is thought to occur. Below the continents, the Moho’s depth can vary from 25 km to 60 km. But underneath the oceans the crust is much thinner, and the Moho lies tantalizingly close to the sea floor—typically just 6 km below it.

In 1957 a group of American geoscientists led by Harry Hess, a founder of the theory of plate tectonics, proposed an ocean drilling...

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