On April 22, 1970, the very first Earth Day, more than 20 million people joined in demonstrations across the United States.
Colleges organized teach-ins to draw attention to the pressing environmental issues of air and water pollution. A part of New York City's Fifth Avenue was shut down to accommodate the marching crowds. Congress recessed so that members could join the days' activities.
Organizers said it was the largest national protest in U.S. history.
Today, Earth Day participants are more likely to carry reusable shopping bags than picket signs. They're more likely to pledge their commitment online than in a line outside city hall.
Forty years later, the tone may have changed, but Earth Day organizers say about 1 billion people worldwide are taking part in the international day of environmental awareness. Many will continue to green their shopping habits, homes, cars and more.
But some environmentalists who remember the first Earth Day -- and the political will that was so palpable then – say they wonder if those individual changes will be enough, considering the massive challenges facing the planet on this Earth Day.
In 1969, the Cuyahoga River in Ohio was so polluted it caught fire. An oil spill off Santa Barbara, California, killed thousands of birds and other marine wildlife. City skies across the country were darkened by smog.
The environmental movement, energized in 1970, helped not only to clean things up but to change the public consciousness.
"There have been advances on almost every front in terms of reduced pollution in our air and water, millions of acres of land protected and parks created, a wide range of new technologies introduced and environmental sensitivities adopted in virtually every walk of life," said Eric Goldstein, director of the Natural Resources Defense Council's New York Urban Program.
In the decade after the first Earth Day, the Environmental Protection Agency was created by executive order and Congress passed the Clean Air Act, the Clean Water Act, the Endangered Species act and other key pieces of environmental legislation.
According to 1997 EPA figures, the number of rivers, lakes and estuaries safe for fishing and swimming doubled in the 25 years after the first Clean Water Act. Smog levels have decreased about 20 percent and lead in the air is down about 90 percent. More than 600 animal species and nearly 800 plant species are listed as threatened or endangered in the U.S.
But the movement itself has changed too.
"The environmental movement in the early days very much rode on the back of the early protest against the Vietnam war," said Sean Miller, education director for the Earth Day Network.
When Sen. Gaylord Nelson, D-Wisconsin, suggested in September 1969 that the country have an Earth Day demonstration to force the environment to the top of the national agenda, Miller said college activists quickly adapted the model of protest they were already using.
But the movement has changed. Instead of local pollution, Miller said, environmentalists today talk of a sustainable society.