Excerpts from envirowonk.com -
The largest accidental oil spill in history happened in 1980 in Mexico. An accident in an oil well caused an explosion which then caused the well to collapse. The well remained open, spilling 30,000 gallons a day into the ocean for a full year. The largest ever was done on purpose in 1991 when Iraqi forces opened the valves of several oil tankers in order to slow the invasion of American troops. The oil slick was four inches thick and covered 4000 square miles of ocean.
The Exxon Valdez was the tanker responsible for the worst oil spill in American history. But it will likely surprise you to know that the Valdez spill was actually only the 34th largest oil spill in history.
The ten largest oil spills, all massively larger than the Exxon Valdez, were all smaller new stories, either because the ships were offshore, or dropped their toxic loads in less developed parts of the world. The Valdez spilled 10 million gallons off the coast of Alaska, the smallest spill in the top ten was four times larger.