Bitumen is a thick, heavy form of petroleum found in Alberta's oil sands (also called tar sands), one of the world's largest petroleum reserves. Unlike conventional crude oil that flows freely, bitumen is so viscous it must be heated or diluted to flow through pipelines. Extracting bitumen requires either mining surface deposits or injecting steam underground to make it flow. Processing bitumen into usable fuels requires additional refining. The oil sands are economically significant for Alberta and Canada but controversial due to greenhouse gas emissions, environmental impacts of extraction, and debates about pipeline construction to transport bitumen to markets.