Gas flaring is the practice of burning off (generally through a tall chimney) gases that cannot easily be captured and put to productive use. It occurs on oil and gas wells, refineries and petrochemical plants, and also landfill sites. Some flaring is still considered necessary at these installations for safety reasons, because it serves as a safety-valve in high pressure systems. However, technological advances over recent decades have made flaring ever less necessary, especially in oil and gas fields, and it has reduced considerably. Nevertheless, the practice is still widespread in some parts of the world—notably, the Niger delta—because it is a cheaper option than investing in the technology necessary to capture, store and transport gases.