The role of lakes in the global methane budget seems to be higher than previously thought. Numerous pockmarks have been described for the marine environment, but at freshwater fluid seeps the geological, chemical and biological processes operating are largely unknown. In Lake Constance, so far more than 450 pockmarks larger > 2m have been identified by side-scan sonar. The pockmarks vary largely in size (dm-range to maximum diameters of 15m) and spatial distribution. Two representative locations (water depth 12m and 85m) close to the former Rhine estuary have been selected for further geochemical examinations. Under-water videos demonstrate continuous release of gas bubbles. Preliminary results of the gas analysis indicate 99% pure methane formed by bacterial acitivtiy rather than a thermogenic origin of the methane. In the sediment the escaping methane gas results in small scale heterogenity of most geochemical parameters. In the water column increased methane concentrations are observed in large areas. The further impact of this massive methane flux from deep reservoir(s) on sediments and water remains to be investigated.