In terms of both its use and composition, crude oil, or petroleum that has been derived directly from the earth, is a very diversified product. The preserved remains of old zooplankton and algae that have gathered in massive proportions on the bottom of the sea (or lake) as a result of anoxic conditions are converted into crude oil. It evolved throughout millions of years from the remains of microscopic aquatic plants and animals that formerly lived in ancient oceans due to the compression and heating of prehistoric biological materials over the course of geological time. The oldest oil-bearing rocks date back more than 600 million years, while the youngest are only a million years old.