What are these rings of coral at the bottom of the Mediterranean sea?

They were discovered by chance in 2011 by Christine Pergent-Martini and her team during a topographic survey mission at sea, the initial aim of which was to study the organisms living off Cap-Corse, at a depth of 120 m (Bonacorsi et al., 2012, 2013).

Circles, all looking identical, around 24 m in diameter gradually appeared on the ship's screens, structures that were unknown to the researchers at the time. A total of 1,400 were counted over an area of around 15 km2.

The Gombessa 6 expedition, led by Laurent Ballesta, diver, naturalist photographer and documentary film-maker, is being organised in collaboration with Andromède Océanologie, the Parc Naturel Marin du Cap Corse et de l'Agriate and the University of Montpellier. A total of three sea trips, four boats and two submarines will be mobilised between 2021 and 2023 for dozens of hours of diving, in order to understand the nature and origin of these rings. The samples collected and the observations made are currently being studied in nearly forty international laboratories by interdisciplinary researchers.

These rings are made up of coralligenous elements (living organisms producing a calcareous structure): coralline algae (Mesophyllum expansum and Lithophyllum stictiforme) and are home to a high level of biodiversity which seem to have adapted to this very special environment (e.g. Callogorgia gorgonia, porcelain, fish, sea urchins and starfish). Their slightly domed structure comprises a central core (1 to 2 m in diameter and 0.2 to 0.5 m high, sometimes eroded) made up of bioconcretions of massive coralligenous algae, an intermediate aureole, made up of biogenic debris and living coralligenous algae, and a peripheral crown (1 to 3 m wide) made up of free calcareous algae (rhodoliths) and invertebrates (sponges, bryozoans and echinoderms). These small, round coralligenous algae structures grow very slowly and have accumulated on the periphery of the rings, following the gentle slope that leads from the heart to the edges. 

Based on the dating carried out on the samples, the climatic (and palaeoclimatic) explanation now seems the most likely. Indeed, over the last few thousand years, the Earth's climate has undergone numerous fluctuations. Glacial-interglacial cycles have occurred every 100,000 years (Lisiecki and Raymo, 2005; Petit et al., 1999). During the last glacial maximum, 21,000 years ago, sea level was around 120 m lower than it is today (Bard et al., 1990; Benjamin et al., 2017). This means that the rings observed today were very close to the surface at that time.

Corals first began to develop at a depth of a few dozen metres during the last glacial maximum, 21,000 years ago. They developed here specifically because the seabed contains very old magmatic rock, interspersed with faults and reliefs, the legacy of the region's tectonic history between 15 and 30 million years ago. With erosion and seawater infiltration, this magmatic rock alters, becoming serpentinite and releasing hydrogen, which can be transformed into methane and escape at the surface of the underwater sediment. Biochemical reactions then lead to the formation of solid structures on the surface of the sediment, favouring the establishment and development of coralligenous algae. Finding themselves trapped below the surface of the sea, they then began to grow in width, following a circular shape, and grew until 18,000 years ago. At that point, the sudden rise in sea level with deglaciation stopped their growth and led to erosion. Paleoclimatic conditions were complex, with tumultuous currents and intense temperature changes. When conditions stabilised again at the end of the deglaciation and the rise in sea level, around 8,800 years ago, the corals began to grow again, right up to the present day.

Corals growing in the middle of an ice age when much of Europe was covered by glaciers? We now know that corals are born and grow outside tropical zones. There are even some that develop in polar environments (Roberts et al., 2006).

Other articles, such as that by Verlaque and Boudouresque (2024), put forward the hypothesis that bombs were dropped in the area during the Second World War. This hypothesis is highly unlikely. First of all, no bombs have been found on the surface of the sediment, the circles are not craters but, on the contrary, have a domed shape, and the publication makes a notable inconsistency, mentioning that the structures would be due to the explosion of bombs when the entire first part of the article mentions a massive dropping of defused bombs. Finally, the site shows no sign of stratigraphic disturbance and the coral core is dated at 21,000 years old.

Bibliography

Documentary Cap Corse, le mystère des anneaux | ARTE, available from April 26th 2025 to August 3rd 2027, Laurent Ballesta and Yann Rineau, https://www.youtube.com/watch?v=R3-1cWGykvk (last visit June 5th 2025).

IMBE Live, Edouard Bard conference, April 30th 2025, https://tv.imbe.fr/la-derniere-deglaciation-et-ses-implications-pour-letude-de-la-grotte-cosquer-et-des-anneaux-de-coralligene-du-cap-corse/ (last visit June 5th 2025).

Bard, E., Hamelin, B., and Fairbanks, R. G.: U-Th ages obtained by mass spectrometry in corals from Barbados: sea level during the past 130,000 years, Nature, 346, 456–458, https://doi.org/10.1038/346456a0, 1990.

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Verlaque, M. and Boudouresque, C. F.: The enigmatic coralligenous atolls of northern Corsica: possible scars from World War II, Scientific Reports of Port-Cros National Park, 38, 165–181, 2024.

June 2025

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