160 km southwest of Reunion Island, just a few dozen metres beneath the surface of the Indian Ocean, lies the peak of an underwater volcanic structure known as Mont La Pérouse. The base of this enigmatic geological formation lies 5000 meters below sea level, with a size comparable to that of Mont Blanc. Permanent currents and strong winds characterise this site in the open seas, culminating in complex diving conditions under which Laurent Ballesta, together with his Gombessa diving team and local researchers had to navigate in order to conduct the study in depth. Further techniques such as observation and photographic inventory, biological and geological sampling, and the use of cameras and sonars were employed in this challenging expedition.