Crude oil—or petroleum—has been used since ancient times for several purposes, mostly for dimly lighting up buildings and streets. Only after inventing the fractional distillation of oil and discovering vast mineral oil fields under the ground and sea in the 19th and 20th centuries, petroleum became the most important material for the modern nations, who could fuel all their cars, airplanes, boats—and, of course, tanks, bombers, and warships that helped make the world safe for more oil consumption.