The site of the Chernobyl Nuclear Power Plant in northern Ukraine has been surrounded for more than three decades by a 1,000-square-mile (2,600-square-kilometer) exclusion zone that keeps people out. On April 26, 1986, Chernobyl's reactor number four melted down as a result of human error, releasing vast quantities of radioactive particles and gases into the surrounding landscape—400 times more radioactivity to the environment than the atomic bomb dropped on Hiroshima. Put in place to contain the radioactive contaminants, the exclusion zone also protects the region from human disturbance.