Milos is a remarkable place to study fluid escape processes. The island’s volcanic history has left behind an active hydrothermal system, where seawater circulates through permeable sediments and fractures, is heated beneath the seabed, and eventually returns to the ocean. In Paleochori Bay, this process is readily visible: gas bubbles rise from the sandy seafloor in waters just five to 15 meters (16 to 50 feet) deep, providing a natural laboratory for investigating how hydrothermal systems work.

Our objective is to test whether we can detect and monitor this gas seepage using distributed acoustic sensing (DAS). DAS transforms an ordinary fiber-optic cable into a continuous array of acoustic and vibration sensors. By sending laser pulses through the fiber and analyzing tiny changes in the light scattered back, we can measure dynamic strain along the cable. In principle, vibrations associated with gas escaping from the seabed could allow us to identify individual seepage areas and investigate how their activity changes over time.

The idea is relatively straightforward. Putting it into practice in a shallow, sandy bay with waves, divers, and hundreds of meters of delicate cable is another matter entirely.

Our first challenge was finding a suitable onshore location for the equipment. Fortunately, we found an unconventional, but excellent, solution: a local beachfront restaurant became the temporary home of our DAS interrogator, the instrument that sends laser pulses into the fiber and records the returning signals. From there, we carefully unspooled the thin yellow cable, guided it across the restaurant terrace, dug a narrow trench through the beach to protect it, and brought it into the sea.

With help from divers and the team onshore, we deployed approximately 500 meters (1,640 feet) of cable across the sandy seafloor, passing close to areas where gas bubbles emerge from the sediment. A stand-up paddleboard helped us manage the deployment at the surface, while divers positioned the cable underwater and secured sections to maintain contact with the seabed. Good coupling between the cable and sediment is important if we want to detect the vibrations produced by hydrothermal activity.

We have also deployed complementary instruments to help us understand what the fiber is recording. At one gas seepage site, an underwater camera and hydrophone are monitoring the rising bubbles. The camera will provide a visual record of bubbling activity, while the hydrophone records the associated underwater sound. By comparing these observations with the DAS measurements, we hope to establish whether variations in the fiber-optic signal can be linked to changes in gas release.

There is still plenty to learn, and this experiment is only a first step. But the possibility of turning a simple cable lying on the seafloor into an instrument capable of listening to hydrothermal activity offers interesting opportunities to study the dynamic processes occurring beneath our oceans.

And it is difficult to imagine a better place to test this idea. Between the beautiful coastline, the extraordinary underwater landscape, and the generosity of our local hosts and collaborators, Paleochori Bay has provided both a fascinating scientific laboratory and a memorable field experience.