Across the globe, mid-ocean ridges have a nondramatic style — nothing like Washington State’s explosive Mount St. Helens or southern Italy’s Mount Vesuvius, the destroyer of Pompeii and Herculaneum. So the team aboard the Meteor wasn’t expecting anything unusual when they passed over a submerged segment of the Mid-Atlantic Ridge called the Reykjanes Ridge. They just wanted to confirm that their equipment was in working order.

The crew switched the equipment on, pinging back X-ray-like images that revealed layers of the seafloor’s stone interior. “We decided to do two test profiles over the Reykjanes Ridge because it was logistically easy and potentially interesting,” said Preine, now a marine geophysicist at the National Oceanography Center in England.

Seismic images of mid-ocean ridges typically show rough and jagged terrain, formed when lava oozes up into the cold ocean along faults or fissures and hardens suddenly into stone. But that’s not what Preine saw. Along the ridge were smooth mounds with steep sides and flat tops, their flanks draped in scattered deposits that looked like debris from an eruption above the sea surface. The formations reminded him of the topic of his doctoral dissertation, a submerged system of notoriously explosive volcanoes near Santorini, Greece.