Scientists are laying a fiber-optic cable inside the crater of Kolumbo, an active underwater volcano near the Greek island of Santorini, turning it into a sensitive undersea "ear" that they hope could one day give early warning of an eruption.
The stakes are high. Kolumbo, which lies about 7 kilometers (4 miles) northeast of Santorini, last erupted in 1650, sending out toxic gases and a tsunami that killed dozens of people on the island.
Today Santorini draws millions of visitors a year. In early 2025, a monthlong swarm of more than 30,000 earthquakes nearby rattled the island, prompted thousands of residents and workers to leave, and exposed how little scientists can see beneath the sea.
The new mission, called HYDROSENSE, brings together the University of Athens, the Monterey Bay Aquarium Research Institute in California and Italy's National Research Council.
Working from the Italian research ship Gaia Blu, the team is lowering about 500 meters (1,640 feet) of cable into the crater, at depths of nearly 500 meters.
The technique, called distributed acoustic sensing, fires laser pulses through the cable and measures tiny changes in the reflected light.
That turns an ordinary cable into hundreds of closely spaced sensors that can pick up faint tremors and even the sound of gas bubbling from the seabed, signals that land-based seismic networks miss.
"Unlike Milos, we cannot rely on divers," said Aaron Micallef, a lead researcher at the California institute, referring to a successful first test in shallow waters off the island of Milos.
Mr. Micallef said robotic submersibles will guide the cable into the steep, rugged crater.
Evi Nomikou, a professor of geological oceanography at the University of Athens, called the technology "a real scientific leap."
The 2025 crisis, she said, made continuous undersea monitoring "a matter of national importance."
The cable will be retrieved when the mission ends. Mrs. Nomikou urged Greece to fund a permanent observatory at Kolumbo.
"It cannot rely only on periodic research programs," she said.