When Reactor Number Four at the Chernobyl Nuclear Power Station blew up on April 26, 1986, the explosion sent into the night an invisible army of unknown strength. Over the next several days tons of radioactive debris and particles fell like silent paratroopers onto fields and backyards and streets not only in northern Ukraine, where the explosion happened, but across a wide swath of Europe. Radioactive isotopes infiltrated the food supply and seeped into groundwater.