Seafloor Telecom Cable Transformed Into Giant Earthquake Detector.

December 2025: Seismic alighting posts are rare on the vast, remote ocean floor. Their scarcity means researchers often can’t detect the first shakings of tsunami-causing earthquakes or the seismic waves that penetrate Earth’s deep interior like x-rays, carrying information that illuminates structures in the mantle and core. But the abyss is home to another kind of technology: the fibre-optic cables that shuttle internet data around the world.

In recent years, researchers have sought to use those cables to supplement ocean-bottom seismometers by watching for shifts in the light coursing through the fibres. Now, a team led by researchers at Nokia Bell Labs has advanced that technique to its ultimate realisation, turning a 4400 kilometre telecom cable linking Hawaii to California into the equivalent of 44,000 seismic stations, spaced 100 meters apart.

The breakthrough, presented on 15th December 2025, at the annual meeting of the American Geophysical Union, has the potential to usher in a new age of imaging the planet’s interior and monitoring the sea floor and the ocean above it. “It’s the instrument we’ve all been waiting for”, says Vala Hjörleifsdóttir, a geophysicist at Reykjavik University who has collaborated with Bell Labs on its early data.

During testing earlier this year the Pacific Ocean cable picked up both the signal of a magnitude 8.8 earthquake that struck the Kamchatka Peninsula in late July and the faint signature of an ensuing tsunami wave as it passed through the ocean and subtly deformed the sea floor. “We’ve seen numerous events”, says Mikael Mazur, an optical-sensing engineer at Bell Labs who led the project, which detailed its early detections in a preprint uploaded to arXiv in September.

The new technique builds off methods developed by Giuseppe Marra, a metrologist at the United Kingdom’s National Physical Laboratory, who earlier this decade devised a way for laser pulses to coexist with internet traffic. Still, questions remain about just how sensitive the approach will be, and whether it will yield usable data, says Andreas Fichtner, a seismologist at ETH Zürich. “It’s not enough to record something. This is high-precision science”.

The method depends on a fibre-optic technique called distributed acoustic sensing (DAS), which scientists have used on land to detect the rumblings of volcanoes and glaciers, and even the footfalls of college marching bands. As light travels down glass fibres, it reflects off randomly oriented defects. When an acoustic or pressure wave, whether from a whale song or an earthquake, crosses the fibre, it stretches and squeezes the defects, causing a phase shift in the light they reflect back to the cable’s source. Measuring those shifts can turn the fibres into a dense array of strain meters. So far, so good—but seafloor cables are interrupted by repeaters, spaced every 75 kilometres or so. The repeaters amplify light for its long journey across the ocean, but dampen the faint back-reflections along that fibre.

Mazur’s team realised that these relays didn’t have to be supposedly showstoppers. The bundles of fibres in each cable resemble divided highways: Light travels out on some fibres and returns on others. But at each repeater there is a “loop-back”, designed to monitor fibre health, that allows light to jump the median, as it were, and travel back on one of the return fibres.

Mazur’s team realised these built-in bypasses enabled researchers to send defect reflections from each stretch back along return fibres, where they would be amplified by repeaters rather than blocked by outgoing ones. With some sophisticated computing, the researchers showed, they could recover reflections even from the farthest sections of the cable creating, in effect, a dense 2D array of transoceanic seismometers.

Scientists eager for the data wouldn’t need their own dedicated fibre—only a laser that piggybacks on the commercial cable, at higher frequencies than the internet traffic. “The beauty of this tech is that it can run on legacy cables”, says Martin Karrenbach, a co-author and geoscientist at Seismic Unusual who has been one of the main figures behind the growth of DAS. “You don’t have to spend hundreds of millions of dollars”.

Fichtner worries rolling out the technology to scientists won’t be so simple. The military might object because the fibre sensors could pick up submarine traffic. Telecom companies might hesitate to tell scientists exactly where their cables are for security reasons. If researchers have to sign nondisclosure agreements to know their locations, other scientists will have no easy way to reproduce their findings. “Those logistical and political problems may be bigger than the technological”, he says.

Still, the potential for a high-resolution view of areas long ignored is enticing, says Verónica Rodríguez Tribaldos, a geophysicist at the GFZ Helmholtz Centre for Geosciences. Such sensors could track whales and monitor ocean currents. They could provide views of tectonic plates as they pull apart in the ocean, or refine how rising plumes of magma in ocean hot spots connect to the base of the mantle. The sensors are there, ready, Hjörleifsdóttir says. “They’re waiting for us to ask what they see”.

Team Maverick.