Clean energy industries may be picking up speed, but there's nothing new about the green movement. On an April day nearly 40 years ago, millions came together to voice their desire for a cleaner future.

Earth Day began as a nationwide grassroots response to growing environmental concerns. Once a date was set, the news services carried the story across the country. Everyone was invited to participate. And they came out in droves.

Twenty million citizens and the thousands of schools and local communities participated on that first Earth Day. President Nixon responded by creating the Environmental Protection Agency.

Dreamed up by Senator Gaylord Nelson and organized by Denis Hayes, Earth Day events helped spawn the modern environmental movement. And from that, the first federal environmental legislation: the Clean Air and Clean Water Acts.

Earth Day is observed each year on April 22 by more than 500 million people and governments in 175 countries.