A quiet transformation is unfolding beneath the oceans. More than 1.48 million kilometres of fibre-optic cables crisscross the seafloor, carrying nearly all global internet and phone traffic. Scientists now believe these same cables can do far more than relay data—they can act as sensors that “listen” to the Earth.
By detecting minute changes in how light moves through them, the cables can pick up subtle shifts in temperature, vibration and seabed movement.
An EU-funded project in the rapidly advancing field of fibre-optic seafloor sensing is developing technologies that could turn the ocean floor into a vast, real-time monitoring system. The discoveries promise new ways to study climate change, better track tectonic activity and strengthen tsunami and earthquake early-warning systems.
A new window into Earth’s hidden depths
Although oceans cover about 70% of the planet, most of the deep seafloor remains almost invisible to conventional monitoring tools.
“We have excellent satellite data for the ocean surface,” explained Marc-André Gutscher, a marine geoscientist at the Geo-Ocean research centre in Brest, France. “But deep down—where most earthquakes and tsunamis begin—we lack direct observations.”
This is starting to change thanks to research into turning existing subsea cables into a global sensing network.
Two main approaches dominate the field: Distributed Acoustic Sensing (DAS) and Brillouin Optical Time Domain Reflectometry (BOTDR). Gutscher led FOCUS, a seven-year EU-funded project completed in September 2025, which explored how these methods can detect tiny, centimetre-scale deformations along active underwater fault lines.
To test the idea, the team deployed a 6-kilometre prototype cable across the North Alfeo Fault near Catania, Sicily—an earthquake-prone region in the shadow of Mount Etna.
Eavesdropping on the ocean floor
In 1908, a magnitude-7.1 earthquake shook the Straits of Messina, sparking a tsunami that killed over 80 000 people—the deadliest natural disaster in modern European history. Understanding the movements of nearby faults could help coastal regions better prepare for such events.
FOCUS partnered with the Italian National Institute for Nuclear Physics (INFN), integrating the prototype cable into its local seafloor observatory. Designed with French fibre-optics company IDIL, the cable resembles standard telecom lines but includes special sensor fibres highly responsive to mechanical disturbance.
Only 9 millimetres thick, it contains both loosely buffered fibres (typical of telecom cables) and tightly buffered fibres that are extremely sensitive to strain. Using BOTDR, the researchers monitored minute changes in fibre length that reflect deformation within the Earth’s crust.
“Our aim was to observe what happens before an earthquake—to catch the early signs of strain before a sudden rupture,” said Dr Giovanni Barreca of the University of Catania.
No major deformation has yet appeared, which is itself revealing. “It suggests the fault is currently locked and accumulating stress,” Gutscher noted. “Whenever that stress releases, we’ll be recording it.”
The prototype has already demonstrated its value. In late 2020, it detected a powerful underwater current—likely caused by a submarine landslide—that travelled across the seafloor. Such events, capable of triggering tsunamis, are rarely captured in real time, and the fibre-optic data provided unprecedented detail.
From the Mediterranean to the Caribbean
The FOCUS project also explored how telecom cables can improve environmental monitoring. Off the Caribbean island of Guadeloupe, researchers used local subsea cable networks to measure temperature changes along the seafloor.
Early work required manual data retrieval every few months from land-based fibre-optic relay boxes. Today, a permanent system allows measurements to be taken remotely every three hours.
Any disturbance—pulling, shifting, warming or cooling—changes the pattern of scattered light in the fibres, allowing scientists to reconstruct what is happening along the cable.
While analysing these light-signal variations, the team recorded a 1.5°C rise in shallow coastal waters over two years, matching satellite data. During the same period, Guadeloupe suffered a severe coral-bleaching event that destroyed roughly 30% of its reefs.
Deeper waters, between 300 and 700 metres, showed smaller warming trends of 0.2–1°C. These results, recently accepted for publication in Geophysical Research Letters, suggest that thousands of kilometres of existing telecom cables could be repurposed to track deep-ocean temperatures—an invaluable addition to climate-monitoring systems.
“While our core interest lies in tectonics, these measurements demonstrate that the very same cables can monitor climate-related changes,” said Gutscher. “The potential for integrated environmental and hazard monitoring is huge.”
A new foundation for global early-warning systems
This approach could be extended to other earthquake-prone areas, including Japan, Cascadia and the wider Mediterranean. DAS can detect the first seismic waves from an earthquake within seconds, while BOTDR can track long-term strain build-up—together offering both rapid alerts and deeper insights into earthquake forecasting.
“The secondary use of fibre-optic cables for sensing is a breakthrough in seismology and hazard monitoring,” Gutscher said. “We’re effectively turning the world’s digital nervous system into an environmental one.”
With continued collaboration and investment, the ocean floor—long hidden from view—could become one of science’s most powerful tools for protecting lives and understanding a rapidly changing planet.
This research was supported by the European Research Council (ERC). The views expressed by interviewees do not necessarily reflect those of the European Commission.
