Many landfill sites are on old quarries of all kinds. The first stage in the construction of a landfill is to dig a hole and shape the sides to approximately 45 degrees. The base of the cell is also sloped into a well. This is to ensure that any liquid drains to a collection point. To ensure that the cell does not leak and that any materials placed in the cell remain there the cell is lined. Firstly, with a layer of clay which also ensures that the sides are smooth. Secondly, with a High Density Polyurethane liner, which is like a thick black bin liner or pond liner. This ensures that the cell is water tight and no liquids leak out into the surrounding land or watercourses. To assist drainage, a layer of gravel is placed in the bottom of the cell. A pipe is then inserted into the well to drain off any liquid produced by the decomposing waste along with rainwater. This liquid is called “leachate” The leachate is piped from the cells to a treatment plant where bacteria render the leachate harmless. Solids are also removed at this point. The treated liquid is then tested before being released into the sewerage system. The test results are inspected by the Environment Agency to ensure the clean-up process is working correctly. When waste decomposes, methane gas is released. Methane is very damaging to the environment as it contributes to global warming as it is 21 times more powerful as a greenhouse gas than carbon dioxide. To prevent the gas escaping, pipes are inserted into the landfill cell. The methane is pumped out of the cell and taken to a power plant where it is used in engines to produce electricity. The cell is now ready to fill with waste. Vehicles arrive daily and tip their waste on an area the size of a tennis court each day. Every night the area is covered over with a layer of soil or similar material to reduce smells, prevent pests getting to the waste and to stop the wind blowing the waste away. It takes about 1 year to 18 months the fill the cell with waste. When the cell has been filled with waste it is capped off, using a further layer of liner. This is then topped with clay and 2 meters of soil. The cell can then be landscaped. Grasses and plants will naturally seed, but trees with shallow roots are planted. This creates a very natural habitat for wildlife. Wildlife often found on restored landfill sites includes foxes, deer, butterflies, dragonflies, frogs and many species of birds including rare ground nesting birds such as skylarks. The relatively undisturbed areas on a restored landfill site are ideal habitats.