Methane is a natural byproduct of microbial decomposition. In lakes, methane accumulates in sediments, especially in the absence of strong disturbance or mixing at the water-sediment interface. Strong disturbance (e.g. fall turnover or poking the sediment with a long stick) may result in dramatic release of the trapped methane… especially in the presence of a flame, as you can see below. Do not try this at home.