Along the Greek coast each summer, tourist boats crowd the shoreline, divers explore the shallows, and beachgoers fill every accessible cove. But just beneath the waterline, something quietly remarkable has been unfolding. Mediterranean monk seals — among the rarest marine mammals on Earth — have been slipping through submerged passages into small, air-filled chambers completely invisible from above.
Researchers had long suspected these animals might be retreating somewhere hidden to escape the seasonal surge of human activity. Now, for the first time, they have the footage to prove it.
A secret room beneath the sea
A bubble cave is exactly what it sounds like: a small, air-filled chamber tucked inside a rocky coastal formation, reachable only by swimming through a submerged passage. From the surface, there’s nothing to see — no opening, no hint that anything lies beyond the waterline. That invisibility, it turns out, is precisely the point.
Scientists first encountered one of these hidden chambers in 2019 on Formicula, an uninhabited islet in the Inner Ionian Sea Archipelago off western Greece. They were installing a camera monitoring system inside a large cave when they noticed an underwater channel extending from the main chamber. Following it, they found a smaller, secondary open-air space — a bubble cave — that also connected via its own submerged passage to the open sea. An accidental discovery, the kind that quietly reshapes a research agenda.
One of the ocean’s most endangered residents
Mediterranean monk seals are among the rarest pinnipeds on Earth. Current estimates put the number of mature individuals between 444 and 600, scattered across the Mediterranean Sea and small pockets of the Atlantic Ocean near northwest Africa. The International Union for Conservation of Nature lists the species as vulnerable, and it’s also protected under the U.S. Endangered Species Act. Numbers are slowly climbing, but the population remains precarious.
Historically, these seals rested on open beaches — exposed and apparently unbothered by their surroundings. Centuries of hunting and human disturbance changed that. The animals gradually retreated to secluded sea caves, trading open shorelines for darker, harder-to-reach sanctuaries. Even that adaptation has limits. During peak tourist season, visitors sometimes enter those caves by boat or on foot, disrupting the very refuge the seals sought out.
Catching them in the act
After identifying the bubble cave on Formicula, researchers installed an underwater camera near its entrance in July 2020. That first device ran for 16 days, capturing images every five minutes. In June 2021, they returned with a more durable camera that operated for 125 days — 141 days of combined monitoring, long enough to reveal a clear behavioral pattern.
During that period, seals visited the large main cave on just 30 days. They visited the bubble cave on 119 days. Both chambers were used simultaneously on 23 of those days. The preference for the hidden, smaller space was hard to dismiss.
The footage held its own surprises. Cameras recorded seals resting on the seafloor, floating at the surface, and engaging in a behavior researchers call “bottling” — drifting vertically in the water column while holding their breath. Some were even spotted bottling upside down.
For co-author Julien Pfyffer, founder of the Octopus Foundation, a Swiss nonprofit involved in the project, the moment the footage came through was unforgettable. “Very quickly, we had this ‘eureka moment’ because we were looking at the factual confirmation of the intuition they might be hiding somewhere,” he told IFLScience. Researchers had suspected the behavior. Now they had proof.
A workaround, not a solution
The study, published April 28 in the journal Oryx, is careful not to overstate what bubble caves represent. Researchers describe them as “less suitable” and “marginal” habitats — a fallback option, not a preferred home. The seals are making do, not thriving in an ideal environment.
That framing carries a clear conservation message. If human disturbance is pushing seals into cramped, hidden chambers, reducing that disturbance could allow them to return to beaches and larger caves far better suited to resting and reproduction. Greece took a step in that direction in 2019, implementing new regulations restricting boating and recreational activity around Formicula.
Marine biologist Jason Baker, of NOAA’s Pacific Islands Fisheries Science Center, put it plainly: “If you protect enough habitat, then they wouldn’t need to do that.” The bubble caves are a symptom. Habitat protection is the treatment.
What this means for protecting the species
The study calls on conservationists to keep beaches and large caves as the primary focus of protection efforts — while also formally incorporating bubble caves into habitat management plans. These secondary refuges are part of how the species is currently surviving, and that matters.
The discovery also reshapes how scientists understand monk seal behavior, revealing a degree of adaptability not fully documented before. That adaptability has limits, though. A species can’t indefinitely improvise its way around mounting human pressure.
How widespread bubble cave use is across the species’ range remains unknown. Formicula is a single data point. Continued remote monitoring at other locations will be essential to determine whether this is an isolated response or something far more common. The seals have been hiding. Scientists are only just beginning to look.