<human>:  When did the Exxon Valdez oil spill happen?
<bot>: The Exxon Valdez oil spill happened in 1989. It happened on March 24, 1989 when the Exxon Valdez ran aground on Bligh Reef in Prince William Sound, Alaska. It was the largest oil spill in U.S. waters. The spill resulted in 11 million gallons of crude oil being spilled into the ocean.