To get serious, you want methane, look at cows.
This source estimates that about 60 percent of methane emissions are human-related.
Our local landfill accumulates and taps off methane generation, and actually sells it to a local company for money. The company uses it to generate electricity for its facilities, and sells the excess back to the grid.
It took a while to debug this. The landfill was finished off, and rebuilt into a nice park. Until they got it all adjusted, sometimes people got small flares here and there in the grass of the park when ignition sources (cigarettes, etc.) got too close.