After a landfill reaches its capacity, the top of it is typically covered with multiple layers of material to prevent people from accessing the waste it contains. The operator ensures that draining systems continue to work, and many landfills capture gas generated by decomposing waste and use it to produce energy.
One of the primary components of a landfill is the drainage system, which is designed to prevent liquid that leaks from the landfill, called leachate, from contaminating local water supplies. Landfills continue to leak this liquid even when they're not in use, so operators continue monitoring the landfill for leaks.
The waste contained in landfills breaks down over time due to anaerobic bacterial reactions, and this process produces a number of gases, included methane, which is the primary component of natural gas. Large landfills often have piping that funnels this gas to a point where machinery can capture it for use as fuel.
It's possible to reclaim the land formerly occupied by the landfill. Freshkills Park, for example, is a project designed to turn a former New York City landfill into the city's largest park. Scheduled for construction through at least 2036, the park is slated to offer nature walking trails, kayaking areas, horseback riding areas and spaces designated for sport and art displays.