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Last week, Marc Chaussidon, director of the Institute of Geophysics in Paris (IPGP), looked at seafloor maps from a recently concluded mission and saw a new mountain. Rising from the Indian Ocean floor between Africa and Madagascar was a giant edifice 800 meters high and 5 kilometers across. In previous maps, there had been nothing. “This thing was built from zero in 6 months!” Chaussidon says.

His team, along with scientists from the French national research agency CNRS and other institutes, had witnessed the birth of a mysterious submarine volcano, the largest such underwater event ever witnessed. “We have never seen anything like this,” says IPGP’s Nathalie Feuillet, leader of an expedition to the site by the research vessel Marion Dufresne, which released its initial results last week.

The quarter-million people living on the French island of Mayotte in the Comoros archipelago knew for months that something was happening. From the middle of last year they felt small earthquakes almost daily, says Laure Fallou, a sociologist with the European-Mediterranean Seismological Centre in Bruyères-le-Châtel, France. People “needed information,” she says. “They were getting very stressed, and were losing sleep.”

The authorities knew little more. Mayotte has a seismometer, but triangulating the source of the rumblings would require several instruments, and the nearest others are several hundred kilometers away in Madagascar and Kenya. A serious scientific campaign started only in February, when Feuillet and her team placed six seismometers on the ocean bottom 3.5 kilometers down, close to the activity.

Data from the seismometers, retrieved by the expedition this month, show a tightly clustered region of earthquake activity, ranging from 20 to 50 kilometers deep in Earth’s crust. The team suspects a deep magma chamber fed molten rock to the sea floor and then contracted, driving the cracking and creaking of surrounding crust. GPS measurements on Mayotte also suggest a shrinking magma chamber: They show the island has sunk by 13 centimeters and moved 10 centimeters east in the past year.

The map of the sea floor, made by the ship’s multibeam sonar, indicates that as much as 5 cubic kilometers of magma erupted onto the sea floor. The sonar also detected plumes of bubble-rich water rising from the center and flanks of the volcano. Feuillet says her team didn’t see the shoals of dead fish that fisherman reported, but they did collect water samples from the plumes. The chemistry of the water will give clues about the composition of the magma, the depth from which it came, and the risk of an explosive eruption.

The crew also dredged up rocks from the flanks of the newborn volcano. “They were popping as we brought them on board,” Feuillet says—a sign of high-pressure gas trapped inside the black volcanic material.