Previous studies have revealed the presence of a widespread Bottom Simulating Reflector (BSR) on the eastern Nankai slope, as well as the occurrence of enigmatic high amplitude reflections that extend well above the BSR. New high-resolution 2-D seismic data were collected on the eastern Nankai slope, during the French-Japanese SFJ cruise in year 2000 and AVA analyses of the enigmatic reflectors are conducted. At the studied location, these analyses suggest that high amplitude anomalies above the BSR delineate the top of gas hydrate rich sediments. Several tens % of the sediment porosity would be filled with gas hydrates between the BSR and a sharp boundary 30 to 60 m above. To account for these observations, we propose that an invasion by free gas of the present day gas hydrate stability zone occurred in the past. Several mechanisms for this intrusion are discussed.