It has been more than a century since any major producer shipped oil in an actual barrel, but the unit has been the industry's standard since the mid-1800s, when overwhelmed Pennsylvania oilmen collected the substance in whiskey barrels after striking their first gushers. Before U.S. drilling began in 1859, "rock oil" (to differentiate it from vegetable oil or animal fat) was sopped up with rags, wrung out and peddled as a cure for everything from headaches to deafness. Spurred by demand for lamp fuel as whale blubber grew scarce, derricks popped up all over Pennsylvania's oil region in the 1860s--although subsequent overproduction drove prices so far down that at one point, a wooden barrel was worth twice as much as the oil it contained, according to Daniel Yergin's definitive tome on oil, The Prize. But as the oil boom took hold and the barrel size was set at 42 gal. (160 L), Pennsylvania's roads became clogged with horse-drawn wagons piled high with the containers, prompting construction of the first oil pipelines (made of wood) and leading 25-year-old John D. Rockefeller to form what became the Standard Oil Co. It would eventually control up to 90% of U.S. oil-refining until the company was broken up in 1911.
Soon after the first long-distance pipelines were laid in the Northeast in the late 1870s and early 1880s, the first oil tankers were allowed to pass through the Suez Canal, and the modern shipping system was born. Today crude oil travels in tankers that can carry up to 4 million bbl. With daily world demand at about 85 million bbl., petroleum represents about a third of all international cargo. And even though the commodity is also measured in kiloliters (in Japan) and metric tons (in Russia), thanks to whiskey, the units are always converted to the 42-gal. barrel for trading and selling.