Landfills are engineered systems that contain solid waste. They may be operated as pits in the ground or filled-in canyons. Regulations of modern solid waste disposal facilities, require that the waste is kept as dry as possible in the landfill, and is isolated from the environment (groundwater, rain, air) in order to minimize environmental pollution. Isolation from the groundwater is accomplished with a bottom and side liner containment system (compacted clay liner, geomembrane, geotextiles). Basic parts of a landfill include the leachate collection system that gathers liquids produced from the waste, the stormwater management system that collects rainwater before it percolates in the waste mass and the gas collection and removal system. Finally, the cover system seals off the top of the landfill to encapsulate the waste mass. A modern landfill has to be carefully designed and well-monitored to ensure compliance with environmental preservation requirements.