Roy Harper recalled watching from the wings as Led Zeppelin took the U.S. by storm in 1973, explaining how the band became a "monster" during the celebrated road trip.

The folk rock icon was part of the entourage known as Zeppelin's "emotional protection unit" and bore witness as the shows got bigger and more powerful, culminating in the three Madison Square Garden concerts that were merged into the movie and album The Song Remains the Same three years later.

"'Unique' is one of those words like 'genius' that is thrown around quite a lot, but Zeppelin were unique," Harper told Uncut in a recent interview. "They pulled a crowd because of the bite, the sheer bite, of that. It became a thing – a walking, talking monster – and as the venues got bigger, they got better, heavier, because they could exercise control."

He continued: "It was a kind of magic and you were blasted into the middle of next week. You paid attention, because attention was being demanded. That wasn't the same as being in England. You couldn't pull that off in the Marquee [Club in London]."

Watch Led Zeppelin Perform 'Rock and Roll' in 1973

In the same feature, famed rock producer and engineer Eddie Kramer shared his memory of taping the Madison Square Garden shows from outside in Wally Heider's Mobile Studio Truck. "I was in the truck with my hands on the fader and all of a sudden it began moving up and down," Kramer said. "It was like an earthquake. The audience was going crazy, cheering and stomping. When you can feel 20,000 people jumping up and down...well that gives you a moment to remember."

Singer Robert Plant reflected that Zeppelin hadn't been certain the tour would prove to be as victorious as it did. "None of us really knew what we were doing," he admitted. "We'd had a great deal of success, but it didn't follow there would be more success as times move on. We wanted to spend time doing it properly and it was time well spent."

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