Marine erosion and intense anthropogenic pressure on the coastal strip combine their effects to permanently damage the tenuous remains left by prehistoric human groups. In the absence of the "destroyer - payer" link that has allowed the development-led archaeology in France since the 1990s, the preservation of this "diffuse" heritage on the coasts is often the sole responsibility of amateur archaeologists. Specific monitoring programmes, such as ALERT (director: M. -Y. Daire), also amplify these collaborative works. Discovered by D. Roué and J.-C. Le Goff, then explorated as part of the Alert project under the responsibility of two of us (GM and POB) in 2015-2016, the Roc'h Santec Leton rock shelter (Santec, Morbihan, France) benefited from these heritage and scientific practices. The rocky island of Roc'h Santec Leton is almost two kilometres from the mainland; The access on foot is only possible a few days a year or by boat. The excavation revealed a very rare stratigraphic sequence in the region : three levels of Middle Palaeolithic, a very damaged Azilian layer and a single layer with diverse dwellings of the Holocene (Early and Late second Mesolithic, Early Neolithic and Iron Age). These multiple cultural components underline the interest of human populations in this rock shelter, now isolated on a tiny island in the middle of the sea. This archaeological operation, which was carried out under difficult logistical and climatic conditions, allowed us to test and validate the principles of intervention, which we believe to have a strong heuristic interest.