Lakes and freshwater systems account for over 20% of all methane emissions into the atmosphere. Until recently, it had been thought that methane could only be produced in oxygen-deprived environ- ments, such as the sedimentary layers found at the bottom of lakes. In reality, surface water, even though it is mixed by the wind and rich in oxygen, produces a signi cant amount of methane, as a group of researchers from the University of Geneva (UNIGE), Switzerland, have recently disclosed. This phenomenon, which is still largely unknown, has been highlighted by a two-year eld study designed to measure the methane concentration and production in the surface water (the top ve metres) of Lake Hallwil in the Swiss canton of Argovia. The UNIGE research, which is published in Nature Communications, shows that lakes play a critical role in the methane gas cycle, where the greenhouse e ect is 28 times higher than for carbon dioxide.