Oceanographers have a saying: Scientists know more about the surface of Mars than they do about the landscape at the bottom of our oceans. But that may soon change. Using data from satellites that measure variations in Earth’s gravitational field, researchers have found a new and more accurate way to map the sea floor. The improved resolution has already allowed them to identify previously hidden features—including thousands of extinct volcanoes more than 1000 meters tall—as well as piece together some lingering uncertainties in Earth’s ancient history. Roughly 90% of the deep-ocean sea floor remains unmapped, a fact that’s been thrown...