Underwater Cameras Capture Seals Resting in Secret ‘Bubble Caves’

Scientists used remote cameras to reveal how some of the world’s rarest seals are resting in secret underwater “bubble caves” to stay out of sight of humans.

According to a report by IFLScience, researchers have identified previously unknown resting spots used by the extremely rare Mediterranean monk seals, a species with only a few hundred individuals left in the wild and considered one of the most threatened seals in the world.

The animals are often seen swimming along the coast of Greece’s Ionian Islands, but they tend to disappear when scientists arrive to observe them resting. Known beach caves were found empty, and early searches in nearby underwater caves did not explain where the seals were going.

To investigate, researchers from the Tethys Research Institute and the Octopus Foundation placed remote cameras in hard-to-reach parts of coastal caves.

They eventually discovered a small submerged passage just beyond a main cave entrance leading into a sealed chamber with a trapped air pocket at the top. When they reviewed the footage, they found the seals were using this space as a hidden underwater “bubble cave.”

“When we discovered remotely that several seals were inspecting the waterproof camera less than an hour after we installed it, we were really amazed,” Julien Pfyffer, study author and president of the Octopus Foundation, tells IFLScience. “Very quickly, we had this ‘eureka moment’ because we were looking at the factual confirmation of the intuition they might be hiding somewhere.”

Footage showed the seals resting in different ways. Some were floating at the surface, others were sleeping in deeper water or lying still on the cave floor. Researchers also saw a behaviour called “bottling,” where seals float upright while resting, sometimes even upside down with their noses underwater.

In a recent study published in the journal Oryx, the researchers detail how Mediterranean monk seals were once commonly found hauling out on open beaches, but in recent years, they have increasingly moved into remote marine caves, likely due to human disturbance. These caves usually have both underwater and above-water entrances, internal pools, and dry areas where seals can rest.

The newly observed “bubble caves” are different because they are fully submerged chambers containing trapped air, accessed through underwater passages. This means the seals can remain in water without needing a dry surface to rest on. Researchers say these spaces may be especially important in busy coastal areas, where disturbance from humans is higher.

“Before this discovery, I believe some scientists had the intuition that Med monk seals were temporarily using these bubble caves, but not to this extent,” Pfyffer tells IFLScience. “Additionally, scientists did not have proof — namely pictures and videos — for such a long time.”