Mediterranean Monk Seals Hide in Underwater ‘Bubble Caves’ to Avoid Tourists During the Busy Summer Season, a Study Suggests

Researchers caught the rare pinnipeds resting and sleeping in a secluded chamber on the Greek islet of Formicula. The findings suggest that these small caves should be included in efforts to protect the animals’ habitat

Sarah Kuta | Daily Correspondent

May 21, 2026 10:40 a.m.

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When summer tourism heats up, Mediterranean monk seals take refuge in hidden underwater “bubble caves”—less-than-ideal hangout spots compared to their typically larger, more accessible caves. The discovery, described in a study published April 28 in the journal Oryx, deepens scientists’ understanding of the species’ behavior and habitat and could help inform conservation efforts.

Mediterranean monk seals are among the planet’s rarest pinnipeds—a group including seals, walruses and sea lions—with an estimated 444 to 600 mature individuals remaining in the Mediterranean Sea and small pockets of the Atlantic Ocean near northwest Africa. Their numbers are slowly increasing, but the species is still considered vulnerable by the International Union for Conservation of Nature and endangered under the Endangered Species Act.

These sleek marine mammals spend much of their time at sea, but they venture onto dry land to give birth and rest. Historically, they perched on open beaches for these activities. But after centuries of human persecution and disturbance, they’ve started hauling out in secluded caves instead. However, even then, tourists sometimes still get too close for comfort by entering the sanctuaries in search of the creatures.

Mediterranean monk seals, it seems, have come up with a clever workaround to this problem. Researchers recently observed them repeatedly using a bubble cave, which is a small, air-filled chamber accessible only via underwater passages—and therefore hidden from humans.

Scientists stumbled upon one of these bubble caves in 2019 on Formicula, an uninhabited islet in the Inner Ionian Sea Archipelago off western Greece. They were installing a camera monitoring system in a large cave when they noticed an underwater channel extending from the main chamber. They realized it led to a smaller, secondary, open-air chamber—a bubble cave—that was also connected via underwater passage to the open sea.

To understand whether and how Mediterranean monk seals were using the bubble cave, the scientists installed an underwater camera near the entrance in July 2020. The device took pictures every five minutes for 16 days. They returned in June 2021 with a sturdier camera, which functioned for 125 days. Whenever seals were present, detected via remotely accessed images uploaded to a server, the cameras started recording video footage.

During the 141 days that the researchers spent monitoring the cave system, the animals visited the main, larger cave on 30 days, the bubble cave on 119 days and both caves on 23 days. That hints at a strong preference for the small, secluded bubble cave.

Scientists who study Mediterranean monk seals had suspected the creatures might be retreating to hidden caves. But they didn’t have any proof, and they didn’t realize the pinnipeds were using them so regularly, reports IFLScience’s Tom Hale.

“When we discovered remotely that several seals were inspecting the waterproof camera less than an hour after we installed it, we were really amazed,” study co-author Julien Pfyffer, founder and chairman of the Octopus Foundation, a Swiss nonprofit collaborating on the project, 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 and snoozing in the bubble cave, either floating on the water’s surface or lying on the seafloor. The cameras even captured them “bottling,” or floating vertically in the water, and sometimes, they were spotted bottling upside down while presumably holding their breath, per the researchers.

The researchers consider bubble caves “less suitable” and “marginal” habitats for Mediterranean monk seals, they write in the paper, but they seem to be serving an important purpose all the same. Conservationists should primarily focus on protecting beaches and larger caves, they argue, but should also factor these secondary refuges into their plans.

Two years ago, Greece’s government implemented new regulations for recreation and boating around Formicula, which should benefit Mediterranean monk seals. In an ideal scenario, these and other protections will allow the seals to rest undisturbed on their preferred beach habitats, rather than relying on caves.

“If the thesis is correct that they’re sleeping in these bubble caves because they want to avoid human disturbance, if you protect enough habitat, then they wouldn’t need to do that,” says Jason Baker, a marine biologist with the National Atmospheric and Oceanic Administration’s Pacific Islands Fisheries Science Center who was not involved with the research, to Science’s Phie Jacobs.

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Sarah Kuta is a writer and editor based in Longmont, Colorado. She covers history, science, travel, food and beverage, sustainability, economics and other topics.