The very first Earth Day was celebrated on April 22, 1970. About 20 million Americans joined rallies, cleanups and teach-ins across the country. Just a few months later, in July 1970, the U.S. government responded by establishing the Environmental Protection Agency.

For the 55th year, the world is using the day to celebrate Mother Nature and the wonderful planet we live on. "Earth Day really is a symbol of the environmental movement," Sarah Davies, the director of communications and media at EarthDay.

The holiday remains one of the largest secular observances in the world, used to spotlight ongoing threats including plastic pollution, deforestation, and global warming. According to EarthDay.org, "more than one billion people now participate in Earth Day activities each year, making it the largest civic observance in the world."