What is the worst environmental disaster in history?

Is it Chernobyl? Exxon Valdez? Global warming? You don't have to think too hard to come up with a long list of candidates for this title, but one far-reaching environmental catastrophe stands above the others.

Our planet has weathered many disastrous events since it banged into being around 4.5 billion years ago. Most recently, Hurricane Katrina, the Chernobyl nuclear explosion and global… » Full Story on HowStuffWorks