About 1,300 strange circles found on the Mediterranean seabed. How did they form?

The unusual structures were first spotted in 2011 during an expedition to survey the seabed. Using sonar, researchers obtained images showing regular circles with dark spots in the center instead of the usual sandy and rocky patches. At first, only a few dozen such formations were found, but it later turned out that there are more than 1,300 of them. They cover an area of several square kilometers and lie at depths of 105–125 meters.

During subsequent expeditions, divers discovered that at the center of each circle sits a dome of red coralline algae. Surrounding it is a light-colored zone of sedimentary deposits, while the outer ring consists, among other things, of small calcareous formations known as rhodoliths.

Thus, these are not simply round depressions in the seabed, but complex biological structures. They serve as a habitat for sponges, corals, invertebrates, and various species of fish.

One of the primary hypotheses attributes their origin to the events of the Last Glacial Maximum, which occurred around 21,000 years ago. At that time, Mediterranean sea levels were considerably lower than today, so areas that now lie at depths of over 100 meters were situated much closer to the surface. This created favorable conditions for the growth of red coralline algae, which require sunlight.

Later, as the climate warmed and glaciers began to melt, sea levels rose, and the former algal colonies gradually ended up at substantial depths.

During a 2020 expedition, scientists collected samples from the central parts of the formations. Radiocarbon dating revealed that the oldest of the analyzed materials originated roughly 21,000 years ago.

According to one hypothesis, about 8,000 years ago, when sea levels stabilized, a new phase in the formation of these structures began. Fragments of ancient formations and rhodoliths gradually accumulated around the central remnants, producing the distinctive circular rings. For now, however, this remains only a hypothesis rather than a conclusively verified explanation.

In 2024, researchers Marc Verlaque and Charles-François Boudouresque proposed an alternative theory. In their view, the circles might be linked to American aerial bombs dumped into the sea between 1943 and 1945 by aircraft returning from combat missions over Europe.

During that period, American air bases were situated near Corsica, and unused ordnance could have ended up in coastal waters. According to this hypothesis, some circular structures could have formed at the sites of bomb impacts or detonations and were subsequently colonized by marine organisms. Thus, their origin might not be tied to ancient geological or biological processes, but rather to human activity in the relatively recent past. However, this version also currently lacks conclusive confirmation.

Regardless of the origin of the circles, scientific research has demonstrated that this section of the Mediterranean Sea holds tremendous ecological value. Numerous marine organisms thrive here, including rare corals and invertebrates.

One of the primary threats to the underwater formations remains shipping. Anchors from large vessels can damage the delicate calcareous structures, which may have taken thousands of years to form.

A portion of the discovered features already falls within the boundaries of the Cap Corse and Agriate Marine Natural Park. Scientists believe that protected status must be extended to the remaining areas in order to preserve this extraordinary underwater ecosystem.

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