Shallow-water marine geothermal systems are widespread in coastal areas and host ecosystems in which microbial communities are supported both by geothermally generated reducing power and by light. The hydrothermal system of Milos island, in Greece, is one of the largest in the Mediterranean Sea. It is part of the Hellenic Arc, whose eastern section reaches the Turkish coast and the island of Kos, and Methana to the west. Extensive submarine venting occurs offshore, from the intertidal zone to depths of more than 100 m, with an approximate extension of 34 km2 of seabed. In Paleochori Bay, the vents are characterized by temperatures ranging from 25 to 119°C and extensive gas and fluid seepage. In May of 2012 we spent ten days in Paleochori Bay, SCUBA diving twice a day to take geochemical mesurements and to collect sediment samples and microbial biomass. These samples are being investigated in the laboratory to understand which organisms are the primary producers and how they fix carbon dioxide.