Nearly 50 years after 20 million Americans participated in the first Earth Day on April 22, 1970, more than 190 countries mark the annual day for raising awareness of environmental causes. And the stakes only grow as the years go by.

Though Earth Day has been dogged by rumors that it was founded by a murderer and as communist propaganda, the truth is much more straightforward — but no less fascinating. TIME spoke to Denis Hayes, a real organizer of the first Earth Day, dubbed “Mr. Earth Day” by the magazine in 1999. Hayes is now the president of the Bullitt Foundation, which doles out grants to environmental efforts.

Here, he tells the true story of founding of Earth Day, its proudest accomplishments and the work that still needs to be done.

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