Biogas is a mixture of gases released when the remains of plants rot in the absence of oxygen. This happens in a natural way in dung heaps, as also in compost plants, garbage dumps, and crops. In large biogas plants, the remains of the plants are decomposed by bacteria in an air-tight tank. The released gas is highly combustible because of its high methane content. The Earth has almost unlimited supply of plant remains, which makes biogas an important source of energy from renewable raw materials. It can be used as gas for heating or for generating electricity. It can also be liquefied easily and used as fuel for vehicles.