Often known as tar sands due to their tar-like appearance and texture, oil sands contain oil particles interspersed with the earth, rather than in oil's standard liquid form. Three significant oil-sand deposits in Alberta, Canada combine to form what is considered to be the largest petroleum source in the world: more than 1.5 trillion barrels of oil mixed in with the earth. (Oil-sand deposits are also found in some other countries, most significantly Venezuela.) Some of the deposits can be reached through surface mining, which requires the removal of vast stretches of earth that contains the oil.
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| Environmental concerns play a role in work at the Canadian tar sands. |
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