On 19th of March 1971, Jethro Tull released their best-selling album Aqualung. It was the fourth studio album by the band and though most people regard it as a concept album, the bandleader Ian Anderson has repeatedly stated that it was "just a bunch of songs" from which only some of them had a unifying theme.
The album was recorded at Island Records' studio in London, at the same time and place that Led Zeppelin were recording their own fourth album. While Tull got the larger of the two rooms, Led Zeppelin were working in the smaller one. In an interview on the 25th anniversary edition of the album, Ian Anderson said that trying to record in their studio room was very difficult, due to its "horrible, cold, echoey" feel.
It was also the first album with keyboardist John Evan as a full-time member and with new bassist Jeffrey Hammond. Prior to this, Evan had collaborated with Tull only to provide several keyboard parts for their previous album, Benefit (1970). Hammond, an old friend of Anderson, had joined the band after their 1970 American tour. Aqualung sold more than 7 million units worldwide
It is a great achievement for its success of combining progressive elements with folk rock sounds and a milestone in Jethro Tull history for its contribution to creating the band's signature sound.