Harbor Porpoises Returning to San Francisco Bay

After an absence of 60 years, Harbor porpoises are returning to San Francisco Bay. They left largely due to pollution from heavy industrial activity during WWII and after. But in the 1970s as a result of the Clean Water Act of 1972, the Bay was cleaned up.

http://news.discovery.com/earth/harbor-porpoise-return-to-san-francisco-bay-111230.html