The first Earth Day, April 22, 1970, attracted over 20 million participants in the United States. It launched the modern environmental movement and spurred the passage of several important environmental laws. It was the largest demonstration in history. People from all walks of life took part in marches, teach-ins, rallies, and speeches across the country. Congress adjourned so that politicians could attend hometown events, and cars were banned from New York's Fifth Avenue.

The event had a major impact on the nation. Following Earth Day, conservation organizations saw their memberships double and triple. Within months, the Environmental Protection Agency (EPA) was created; Congress also revised the Clean Air Act, the Clean Water Act, and other environmental laws.

The concept for Earth Day began with Senator Gaylord Nelson, a Wisconsin Democrat, who in 1969 proposed a series of environmental teach-ins on college campuses across the nation. Hoping to satisfy a course...