Why is the small bubble at the bottom of the pond biogas, which is in the ninth grade? urgent

This is the knowledge of high school chemistry. It is a gas called methane, which is formed by the decay of plants in the absence of air.

The green plants in the pond, with leaves and green stems, will fall into the water in autumn and be deposited in the water under the action of gravity. If the water is moving, it may drive the soil to move and bury the fallen plants under the soil.