In 2011, during a mapping campaign conducted off the coast of Cap Corse by IFREMER (French Research Institute for Exploitation of the Sea), a thousand strange and perfectly circular formations, regularly outlined on the seabed between 115 and 140 m, appear on the monitoring screens. Never explored, these immense coralligenous atolls (each one 30 m in diameter) are a great mystery. What is their origin? What are the reasons for their presence in the Mediterranean?