Is the Exxon Valdez spill site finally clean?

Next year will mark the 20th anniversary of the Exxon Valdez oil spill - perhaps the most notorious human-caused environmental disaster in history.

But, according to the latest survey of Prince William Sound in Alaska - where the oil tanker foundered in 1989 - very little oil remains and most of what does is not in a form or location that can harm animals, plants or humans.