Petroleum was formed from the remains of tiny sea plants and animals that died millions of years ago. When the plants and animals died, they sank to the bottom of the oceans. Here, they were buried by thousands of feet of sand and silt. Over time, this organic mixture was subjected to enormous pressure, and heat as the layers increased. The mixture changed, breaking down into compounds made of hydrogen and carbon atoms--hydrocarbons. Finally, an oil-saturated rock-much like a wet household sponge was formed.