The first ten years of “The Tonight Show Starring Johnny Carson,” recorded on reel-to-reel videotape, are almost all gone, recorded over by NBC.

In 1972, Carson’s contract negotiation included ownership of the show and he took pains to have the next 20 years of shows, which include more than 3,300 hours of television, stored in a secure and temperature-controlled environment: A once active salt mine in Hutchinson, Kan.

To filmmaker Peter Jones, finding out about the treasure trove seven stories underground was a eureka moment. “When I found out that all the tapes were stored 650 feet underground in Hutchinson, Kansas, I said, ‘That’s my opening to the film! In this salt mine!’ ”

Are the tapes still down there? “Yes,” says Jones. “They’ve been there for many years. Johnny found a place where they could be safe and sound. And when they were digitized a few years ago, they were pulled from the salt mine and then returned [there]. But in those tapes are all the clues! We’ve found clips that have never been seen since the night they were broadcast. People don’t remember Johnny talking about himself on the show, but he did. And he did it there in a way that he never could do in other places.”