This oil spill in Santa Monica was the worst in U.S. waters at the time.

The weekend oil leak along the Southern California coast happened not far from the site of the catastrophe more than a generation ago that helped give rise to the modern environmental movement itself: the 1969 Santa Barbara oil spill.

Thursday will be the 51st Earth Day, an event celebrated by more than one billion people in 193 countries worldwide, currently coordinated by the Earth Day Network.