The dome-like mounds poking up in sonar maps of the seafloor caught scientists’ eyes. They stood out in stark contrast to the surrounding environment off the coast of Santa Barbara, Calif.

“They came up very suddenly out of the seafloor: There were seven of them. The largest we called Il Duomo, and it is about the size of two football fields side by side and as tall as a six-story building,” said David Valentine, an earth scientist at the University of California, Santa Barbara.

“Nobody knew what the domes were made of,” said Chris Reddy, a marine chemist at Woods Hole Oceanographic Institution.

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