During July 2011, a submarine “gas explosion”, consisting of gas, water and seaweed, occurred at the area NNE of Riva port in Thirassia island, part of the Santorini Volcanic group in the Central Aegean Sea. Scuba diving revealed an elliptical crater with intense gas leakage at the flat area at the depth of 37.5m, with a diameter of approximately 1.5 m and height estimated around 1-2m. It was covered by a white substrate, a few centimeters in thickness, while at the deepest part of it sulfur deposits of pale yellow color prevailed. Mucilaginous aggregates were clearly distinguished in the seawater samples collected. The relatively high temperatures (19-21°C) combined with chemical changes on the environment due to geological activity, have been most possibly the triggering factors for the slime production.