Petroleum, (from Greek πέτρα - rock and έλαιο - oil) also called crude oil, is a thick and black liquid. It is a natural material mainly made of hydrocarbons. Most petroleum is found by drilling down through rocks on land or off-shore on the continental shelf. Major producers are in the Middle East, the Americas, and Russia. It is the most important world fuel source. It supplies 38% of the world's energy and is also used to make petrochemicals.