Nearly 50 years ago, about 20 million people took part in the very first Earth Day celebration in cities, towns and college campuses across the U.S. They were taking a stand against pollution and promoting clean air, water and earth. (Considering that the U.S. population was 205 million at that time, that’s an impressive participation rate, translating to roughly one in 10 Americans.)

The man behind the first Earth Day celebration on April 22, 1970, was Gaylord Nelson, a U.S. senator from Wisconsin. In some ways, his inspiration for the nationwide Earth Day event was a successful culmination of his nearly decade-long quest to get his fellow congressmen to make pollution control and conservation a higher priority. He played a large role in the passage of conservation bills in Congress, including the Wilderness Act and the National Wild and Scenic Rivers Act. However, other efforts, including a 1963 speaking tour about pollution and conservation (with John F. Kennedy as the keynote speaker), and his first bill to ban DDT, failed to spark a sufficiently large movement to hold the attention of Congress.

But in the fall of 1969, when Nelson had the inspired idea to create Earth Day, the time had come when people were ready to hear the message.

At that time, after nearly a decade of activism, concerns about Industrial pollution began to reach critical mass. One event seemed to perfectly illustrate the need for industry to change its practices, and that was the 1969 Cuyahoga River fire in Cleveland. By today’s standards, a river fire caused by industrial dumping would be seen as a major failure of industrial practices and government oversight.

Yet at that time, a river fire in an industrial city caused by pollution wasn’t unheard of. In fact, this wasn’t the first time the Cuyahoga River burned. But by the late 1960s, the idea of reforming industrial practices was starting to gain traction.

In late 1969, Nelson was returning from touring the damage from a different industrial incident: an oil spill in California. On the airplane ride, he had a light-bulb moment. Inspired by the anti-war teach-ins held across the country at college campuses, he wondered if a massive environmental teach-in would get Congress’s attention — especially if all the teach-ins were held at various college campuses across the country, all on the same day. He shared the idea with a Seattle conservation group and an Atlantic City auto workers group, and their conversation was widely reported by the news media. People read the stories and called Nelson’s office, asking how they could get involved with organizing a local event.

From there, Nelson set up a steering committee and a nonprofit to serve as a resource for organizers, and the grassroots effort took off.

On the first-ever Earth Day event, which was April 22, 1970, events across the U.S. were as diverse as the organizers. Some were what you might expect, featuring speeches, workshops, theatrics and pamphlets. Madison, Wisconsin, began its multi-day event on the university campus with a sunrise ceremony, with readings from environmentalists, Henry David Thoreau and the Bible. Albuquerque students marched on the grounds of sewage plants for better odor control. A mother in Philadelphia led bus tours to spotlight the smoke refineries discharged in her neighborhood. New York students pulled discarded appliances and tires from the Hudson River.

The 20 million people who took part in the first Earth Day grabbed the attention of the media as well as lawmakers. Over the next decade, Congress passed several new environmental regulations, such as the Endangered Species Act, the Safe Drinking Water Act and the Toxic Substances Control Act, and it also strengthened existing regulations, including the National Environmental Policy Act and the Clean Air and Water Acts.

Earth Day celebrations of today have a different flavor from the original 1970 event. You’ll still see many turning out to pick up trash at parks and roadsides, but some events might have a larger emphasis on lifestyle changes, such as the tiny house movement, sustainable food and renewable energy.

But at its heart, the overall message of Earth Day is much the same: We have but one planet, and it’s our call to take care of it.