The signature power ballad on Led Zeppelin IV towers over Seventies rock like a monolith. From the Elizabethan ambience of its acoustic introduction to Plant's lyrical mysticism to Page's spiraling solo, the eight-minute song is a masterpiece of slow-reveal intensity that withholds power, then ascends skyward like nothing in rock. "It speeds up like an adrenaline flow," said Page, whose on-the-spot improvisation was the perfect complement to Plant's evocation of excess and salvation. "It was a milestone for us."