On Earth Day 1970 20 million Americans displayed their commitment to a clean environment. It was called the largest demonstration in human history, and it permanently changed the nation's political agenda. By Earth Day 2000 participation had exploded to 500 million people in 167 countries. The seemingly simple idea—a day set aside to focus on protecting our natural environment—was the brainchild of U.S. Senator Gaylord Nelson of Wisconsin.