Petroleum is a thick, dark liquid found deep underground that formed from the remains of ancient plants and tiny sea creatures that died millions of years ago. Over enormous spans of time, heat and pressure transformed these buried organisms into the oily substance we drill for today.

Petroleum is one of the most important natural resources in the modern world. When refined in special factories called refineries, petroleum separates into different products: gasoline that powers cars, diesel fuel for trucks and trains, jet fuel for airplanes, and heating oil for homes. It also becomes the raw material for plastics, which means petroleum is in countless everyday objects, from your toothbrush to your computer keyboard to the soles of your sneakers.

Because it takes millions of years to form, petroleum is called a fossil fuel. Once we use it up, it's gone for good, which is why scientists and engineers are working hard to develop alternative energy sources for the future. Finding petroleum deposits requires special equipment and knowledge, and extracting it from the ground (or sometimes from the ocean floor) is complex, expensive work.