Shale gas is natural gas produced from shale. Because shale has matrix permeability too low to allow gas to flow in economical quantities, shale gas wells depend on fractures to allow the gas to flow. Early shale gas wells depended on natural fractures through which gas flowed; almost all shale gas wells today require fractures artificially created by hydraulic fracturing. Since 2000, shale gas has become a major source of natural gas in the United States and Canada.Because of increased shale gas production, the United States is now[when?]the number one natural gas producer in the world.Following the increased production in the United States, shale gas exploration is beginning in countries such as Poland, China, and South Africa.