Chernobyl, Ukraine

Number of people potentially affected: Initially estimated at 5.5 million, currently disputed
Type of pollutant: Radiation
Source of pollution: Nuclear meltdown

When Chernobyl melted down on Apr. 26, 1986, the ruined plant released 100 times more radiation into the air than the fallout from the nuclear bombs at Hiroshima and Nagasaki. Today the 19-mi (30-km) exclusion zone around the plant remains uninhabitable, and between 1992 and 2002 more than 4,000 cases of thyroid cancer cases were diagnosed among Russian, Ukrainian and Belarusian children living in the fallout zone. "It's the largest industrial accident in the world," says Fuller. "It'll be contaminated for tens of thousands of years." Fortunately, work is being done to prevent further radiation spill from the ruined sarcophagus of the nuclear plant.

— by Bryan Walsh