This is what we found on Wikipedia regarding the Zeppelin Unledded:

The untitled fourth studio album by the English rock band Led Zeppelin, commonly known as Led Zeppelin IV, was released on 8 November 1971, by Atlantic Records. It was produced by the band's guitarist, Jimmy Page, and recorded between December 1970 and February 1971, mostly in the country house Headley Grange. The album contains one of the band's best-known recordings, the eight-minute-long "Stairway to Heaven".

The informal recording environment inspired the band, allowing them to try different arrangements of material and create songs in various styles. After the previous album Led Zeppelin III (1970) received lukewarm reviews from critics, they decided their fourth album would officially be untitled and represented instead by four symbols – one chosen by each band member – without featuring the name or any other details on the cover. Unlike the prior two albums, the band was joined by guest musicians: singer Sandy Denny on "The Battle of Evermore", and pianist Ian Stewart on "Rock and Roll". As with prior albums, most of the material was written by the band, though there was one cover song, a hard rock re-interpretation of the Memphis Minnie blues song "When the Levee Breaks".

Led Zeppelin IV was an immediate critical and commercial success and is Led Zeppelin's best-selling album, having sold over 37 million copies worldwide. It is one of the best-selling albums in the United States and of all time, while critics have regularly placed it high on lists of the greatest albums of all time.