Most municipal sewage is treated by aerobic biological process (called activated sludge) and the
created sewage sludge further treated (stabilized to prevent odors) under either aerobic or
anaerobic conditions, the later
creating methane gas that can be used.
Thus, the craters discussed here might originate from cave pond formation, due to permafrost degradation, which may be
created conditions for the accumulation of an explosive air / gas mixture, because of the contained area and
anaerobic conditions, which favor methanogenesis.