What does a mountaineer do once he summits the highest peak? He dives off and learns to fly. Such was the situation, and solution, of Led Zeppelin’s members in 1972 as they planned their follow-up to Led Zeppelin IV. This prior album needs no introduction, being one of the great critical and commercial successes of the age (or ever, for that matter). But instead of going back to the blues rock/heavy metal sound that had vaulted them to international fame, they chose to spread their wings, step off the cliff, and pray they wouldn’t come crashing to earth.