Crude oil and natural gas are naturally occurring substances that are found in rock in the Earth's crust. These organic raw materials are created by the compression of the remains of plants and animals in sedimentary rock such as sandstone, limestone, and shale. Companies in this industry are involved in the exploration and production of crude petroleum, mining and extraction of oil from oil shale and oil sands, exploration and production of natural gas, sulfur recovery from natural gas, and recovery of hydrocarbon liquids.