During the 1960s, hundreds of thousands of young Americans rejected the stable, comfortable middle-class life their parents had built in the years after World War II, driven instead by a spirit of rebellion that would leave a lasting impact on the nation.

But long hair and beards for men, tight bell-bottom blue jeans and flower crowns for women, and the widespread use of mind-altering drugs were only the most visible, and easily dismissed, signs of this ‘60s “hippie” counterculture.

Far more transformative were the radical social and political movements that many of its adherents embraced, including the civil rights movement, the movement to oppose the Vietnam War and—at the tail end of the 1960s and the outset of the 1970s—the environmental movement.

Rachel Carson’s bestselling book Silent Spring, published in 1962, introduced many Americans to the devastating effects of the large-scale use of pesticides, especially DDT. As the 1960s continued, more and more people became aware of other threats to the environment, such as automobile emissions, oil spills and industrial waste.

By 1967, the federal government had passed the first Clean Air Act, the first federal emissions standards and the first list of endangered species (including the bald eagle, America’s national symbol). These laws were a start, but they did not go far enough to address the serious environmental problems facing the nation.

In January 1969, the Union Oil well in Santa Barbara, California spilled more than 200,000 gallons of oil into the Pacific Ocean over 11 days. That June, oil and chemicals floating on the surface of the Cuyahoga River in Ohio burst into flames. Images of such disasters, broadcast across the country, helped fuel a growing outrage over the state of the environment, especially among young radicals.

Despite this growing consciousness, environmental activists hadn’t yet come together as a true movement by the end of the 1960s, as civil rights and anti-war activists had. This lack of momentum had long frustrated Gaylord Nelson, a Democratic senator and former governor of Wisconsin (1959-63) who was one of Congress’ most passionate environmentalists. During his years in the Senate, Nelson had also backed civil rights legislation and voted against appropriating funds for the war in Vietnam.

In August 1969, Nelson traveled to California, where he spoke at a water conference and visited the scene of the Santa Barbara oil spill. On that trip, he was struck by an article he read in Ramparts magazine about the anti-war “teach-ins” held on college campuses in the mid-1960s. Though teach-ins had been abandoned as an anti-war tactic, Nelson now saw their potential to energize people—especially young people—by educating them about the need to protect the environment.

On September 20, 1969, speaking at the annual symposium of the Washington Environmental Council in Seattle, Nelson announced that he was planning a nationwide teach-in on the environment for the following spring. “I am convinced that the same concern the youth of this nation took in changing this nation’s priorities on the war in Vietnam and on civil rights can be shown for the problem of the environment,” he said.

To put his plan into action, Nelson reached across the aisle in Congress, recruiting the Republican congressman Pete McCloskey of California to serve as his co-chair on the steering committee behind the event. Despite his otherwise conservative views, McCloskey was a committed environmentalist who also opposed the Vietnam War.

In December 1969, Nelson hired Denis Hayes, the 25-year-old former president of the student body at Stanford University, as national coordinator of the Environmental Teach-In, as Earth Day was originally known. On a tight budget, Hayes recruited a small staff of volunteers, many of them students, to come to Washington, D.C. and coordinate Earth Day events in various regions of the country.

Thanks in large part to these committed young grassroots activists, the first Earth Day took place on April 22, 1970. In New York, 250,000 people flooded Fifth Avenue, after Mayor John Lindsay agreed to bar traffic for two hours between 14th and 59th Streets, all the way up to Central Park. In Miami, supporters of Eugene McCarthy, the anti-war presidential candidate in 1968, staged a parody of the Orange Bowl parade called the “Dead Orange Parade.” As Adam Rome recounted in his book The Genius of Earth Day, one of the parade’s floats featured the Statue of Liberty wearing a gas mask, standing on a pedestal made out of garbage.

Though these urban events made the biggest splash in the press, the true impact of Earth Day would come from the more than 12,000 events scattered around the country, attended by an estimated 20 million Americans. Many were held at high schools and colleges, and they featured more than 35,000 speakers, from scientists to folk singers to members of Congress, which had adjourned for the day.

Earth Day’s success helped spur long-delayed action in Washington on behalf of the environment. Just eight months later, Congress authorized the creation of the Environmental Protection Agency (EPA), and the 1970s would see passage of a slew of environmental bills, including the Clean Air Act of 1970, the Clean Water Act of 1972 and the Endangered Species Act of 1973.

At the same time, colleges across the country established environmental studies programs, aiming to harness the wave of youthful energy for the future. Environmentalism may have begun as a countercultural force, but Earth Day made it into a movement.