<human>:  When did the Exxon Valdez oil spill happen?
<bot>: The Exxon Valdez oil spill happened in 1989. It was a huge oil spill that occurred off the coast of Alaska in Prince William Sound. It was caused by a tanker ship named the Exxon Valdez, which had been transporting oil from Alaska to a refinery in California. The ship hit a reef and spilled millions of gallons of oil into the water, killing many animals and harming the environment.