The Alan Parsons Project are a British Progressive Rock band that have been around for over 40 years, enjoying moderate success throughout the mid to late 70s. In 1979, the band was exhausted from constant touring and studio work and wanted a break. Their label, Arista Records, however, heavily disagreed and decided to set due dates for the band's 3rd and 4th albums. Wanting to get everything over and done with, the band decided to record both albums at once.
The first of the two albums, Eve, was released as scheduled. The second, titled The Sicilian Defense (named after a move in chess) was a different story altogether. It was slapped together lazily and poorly written on purpose as to figuritively raise the middle finger to Arista. The music was allegedly atonal, inaccessible, and virtually unlistenable. Arista Records were so horrified by the results that they stashed it away in their vaults, never to be released.
Most of the tracks have never surfaced. Fans were clammoring for decades for the album's release. Parsons, as a joke, included a single track on the bonus disc on a reissuing of Eve in the 2000s. After the fans figured out just how bad the music was, the clammoring stopped. Though, Parsons has said on several occasions that he hopes the album never sees release he announced that a Alan Parsons Project box set was in the works and was to contain the album in its complete form (much to the endless groaning of fans). This does not, however, render the album as not missing, as Parsons is notorious for canceling plans for such releases and the announcement may have just been a prank. Until the disc is released, the album is still "missing".
Update 3-10-2014: The album is now getting an official release on March 31st in a boxset chronicling all of the Project's albums. While the disc set is pricey, don't think that's going to stop the internet from leaking it. It will be interesting to see if the album is really as bad as Parsons thought.