The U.S. oil industry pumps more than 3 billion barrels of crude per year.Oil crosses continents in pipelines like the Keystone, which moves 1.3 million barrels per day.

It travels between them on tanker ships, the largest of which can carry 3.7 million barrels.

Oil Barrels 1a Supri Supri ReutersWhen oil leaks, the disaster is quantified in barrels spilled—more than 250,000 from the Exxon Valdez, and at least 3 million from Deepwater Horizon.

When oil sells, it is priced per barrel, and when it burns, its energy output is measured in “barrel-of-oil equivalents” (5.8 × 106 BTUs). The world of oil is a world of…