Happy Earth Day! Celebrate With Google Doodles [PICS]

Happy Earth Day! April 22 is the anniversary of the modern environmental movement, dating back to 1970.

With a billion people a year marking the occasion, it's described as the "largest civic observance in the world." Boasting 22,000 partners in over 190 countries, this year the Earth Day Network is gearing up to celebrate "A Billion Acts of Green."

Since 2001, Google has marked each and every Earth Day with a specially-designed Doodle adorning its famously sparse homepage. To celebrate this special occasion, we've gathered them all into a gallery showing all of the Doodles.