Leisure suits, hot pants and patched jeans. Disco, Datsuns and Corvairs. The Bicentennial, wide ties and cassette tapes.

The ’70s. The weird, in-between decade — part ’60s, part ’80s. A mix of optimism, pessimism and cynicism. An American stew of smog, Watergate and Earth Day.

It’s all history now, unheralded, in a way, but the National Archives is looking to change that with a striking new exhibit that provides, in scores of color pictures, a portrait of the United States in the ’70s.

Titled “Searching for the Seventies: The Documerica Photography Project,” the exhibit is based on an archive of 22,000 photos that were taken by the fledgling Environmental Protection Agency in the 1970s and then forgotten.

The project was supposed to document environmental problems of the time, but wound up illustrating the broad fabric of life across the country.

This 1972 photo of a hitchhiker with his dog, “Tripper,” on U.S. 66 is part of the Searching for the Seventies: The Documerica Photography Project. (The U.S. National Archives)

The exhibit, which features about 90 photographs, opens Friday and runs through Sept. 8 in the Lawrence F. O’Brien Gallery in the Archives building in Washington. There are hundreds more online.

The photos do capture the horrors of pollution:

Gutted automobiles sitting in a pond filled with acid-and-oil tainted water near Ogden, Utah, in 1974; children swimming against the backdrop of a chemical plant in Lake Charles, La., in 1972; a small house in Poca, W.Va., with laundry on a clothesline and power-plant cooling towers in the background in 1973.

But they also show off the decade’s people:

Two young women, their arms entwined, posing against a graffiti-covered wall in Brooklyn; an African American couple — he in a powder blue suit, she in a red dress, white shoes and hat — in Chicago in 1975; a Colorado rancher in cowboy hat and denim jacket against a snow-covered backdrop.

There’s also a picture of an abandoned gas station turned into a church in Potlatch, Wash., in 1974. “Fill up with the Holy Ghost & Fire,” it says on one of the old pumps outside.

Another shot depicts a grimy, forlorn-looking, unemployed African American man sitting on a windowsill in Chicago in 1973. And another shows an auto dump in California in 1972, with layers of crushed Chevys, Cadillacs and Fords.

“As you walk through the exhibition,” David S. Ferriero, archivist of the United States, said Tuesday, “those of you of an age will recall your own memories of this era — the amusing, the embarrassing, the life changing.”

“For some of us, seeing the record of our . . . somewhat recent memories on museum walls can be a bit dismaying,” he said. “But that’s part of the march of history.”

Jim Gardner, executive for Legislative Archives, Presidential Libraries, and Museum Services, said the exhibit “explores a decade that we tend to overlook, when we’re not rolling our eyes at the very thought of it.”

“No one would be surprised if we did an exhibit on the ’60s, and we can all imagine that ourselves,” he said. “But the ’70s is a very different thing.”

The collection of photographs resulted from the work of the late Gifford Hampshire, a child of the Dust Bowl and a World War II veteran who was hired as a press officer by the EPA and directed the original Documerica project.

The little-known Documerica photos were noticed about 15 years ago by curator Bruce I. Bustard, who found them in the Archives and realized they would make a great exhibit. He interviewed Hampshire before he died in 2004.

“This was kind of his life’s work,” Bustard said Tuesday. “He was prouder of this than anything else he had done in his career. Unfortunately, the pictures were kind of lost for a long time.”

Hampshire, who had been a photo editor at National Geographic magazine in the 1950s, was heavily influenced by the famous photographs of rural America taken by the Farm Security Administration in the 1930s and ’40s.

He was born in Iowa and had grown up in Liberal, Kan. “He was very much affected by the Depression,” his daughter, Victoria, said Tuesday. He wanted to do a similar project for the 1970s as had been done for the ’30s and ’40s.

Hampshire hired scores of freelance photographers, at $125 a day, and sent them across the country to chronicle what they saw. There were a lot of “wayward photographers sleeping over at the house,” Victoria Hampshire said of her childhood home, a farmhouse in Fairfax County.

“My father was so excited about this project,” she said at the Archives. “This really was his baby, his brainchild. . . . He felt that it was critical to leave some kind of documentary of the United States.”

“He would be absolutely floored if he knew this was happening today,” she said. “He didn’t feel [the original project] was appreciated. . . . It got basically stuffed in the archives.

“It was hard for him. He saw what he thought was a very valuable project just be stuffed away in the dust,” she said. “I don’t think he ever felt that it served its full purpose. And I think it will now.”