It naturally seeps to the Earth's surface along fault lines and cracks in rocks, where it can contaminate water as tar, asphalt or bitumen deposits.
Crude oil ranges in color from almost clear to green, amber, brown or black.
It may flow like water or creep like molasses. The plankton died and drifted downward, mixing with sediments of mud and silt. As the centuries passed, layers of sediment covered the decaying animals and plants, burying them deeper and deeper. The pressure and temperature rose, transforming the plankton into oil. Oil is critical for the U.S. economy and the conveniences of our everyday lives.