Petroleum, also known as crude oil, is a naturally occurring liquid found beneath the earths surface that can be refined into fuel. It is a fossil fuel that was formed from the remains of ancient marine organisms that lived millions of years ago in a marine environment before the existence of dinosaurs. Over millions of years, the remains of these animals and plants were covered by layers of sand, silt, and rock. Heat and pressure from these layers turned the remains into what we now call crude oil or petroleum.

Petroleum is composed of hydrocarbons, which are mainly hydrogen (about 13 percent by weight) and carbon (about 85 percent) . Other elements such as nitrogen, sulfur, oxygen, and metals such as iron, nickel, and copper can also be mixed in with the hydrocarbons in small amounts. Petroleum is used to make gasoline, an important product in our everyday lives. It is also processed and part of thousands of different items, including tires, refrigerators, life jackets, and anesthetics.

When petroleum products such as gasoline are burned for energy, they release toxic gases and high amounts of carbon dioxide, a greenhouse gas. Carbon helps regulate Earth's atmospheric temperature, and adding to the natural balance by burning fossil fuels adversely affects our climate. Because the majority of the world relies on petroleum for many goods and services, the petroleum industry is a major influence on world politics and the global economy.