Jethro Tull were a unique phenomenon in popular music history. Their mix of hard rock, folk melodies, blues licks, surreal, impossibly dense lyrics, and overall profundity defied easy analysis, but that didn’t dissuade fans from giving them 11 gold and five platinum albums. At the same time, critics rarely took them seriously, and they were off the cutting-edge of popular music by the end of the ’70s. But no record store in the country was complete without their 1971 classic  Aqualung .