Democratic senator Gaylord Nelson of Wisconsin suggested the idea of Earth Day "to shake up the political establishment and force this issue [environmentalism] onto the national agenda." As a result, on April 22, 1970, some twenty million people participated in grassroots demonstrations all over the country. This banner displayed on the Mall in Washington, D.C., focused on clean air. Other activists dramatized oil spills, toxic dumps, pesticides, polluted rivers and lakes, the loss of wilderness, and the extinction of wildlife. Dennis Brack/Black Star/Stockphoto.com.