The oldest method of waste disposal for the solid matter discarded in the domestic dustbin, along with the packaging material and paper from high street shops and offices. Landfill sites are usually disused quarries and gravel pits. When they were filled, previous practice was to cover them up with soil and forget about them. Housing estates have been built, often with disastrous consequences, on old landfill dumps. Waste burial has now become a serious technology and a potential source of energy. Landfill sites can be designed to be bioreactors, which deliberately produce methane, gas as a source of biofuel or alternative energy. Traditionally, waste tips remained exposed to air and aerobic microbes - those which thrive in air - in order to turn some of the waste into compost. However, open tips also encourage vermin, smell in hot weather and disfigure the landscape. In the 1960s, as a tidier and safer option, landfill operators began to seal each day's waste in a clay cell. While excluding vermin, the clay also excluded air. Decomposition relied on anaerobic microbes, which die in air. However, the process produced methane (natural gas), which was a safety hazard. The methane is now extracted by sinking a network of perforated pipes into the site.