From wikipedia (entry has been updated to include Giles Martin's recent comments) https://en.wikipedia.org/wiki/Carnival_of_Light
"Carnival of Light" is an unreleased experimental piece recorded by the Beatles on 5 January 1967 for "The Million Volt Light and Sound Rave", an event held at the Roundhouse Theatre on 28 January and 4 February 1967. The track is nearly 14 minutes long and contains distorted sounds of percussion, keyboards, guitar, vocals, and various effects. The piece has not yet appeared on any official Beatles release. In 1996, McCartney tried to release the track on the compilation album The Beatles Anthology 2, but George Harrison voted to reject it. The track was confirmed by McCartney to be in his possession in 2008, but his attempt to release it to the public has been unsuccessful. As of 2016, he was still considering the track's release. Recording and mix Beatles expert Mark Lewisohn, who listened to the song in 1987 while compiling his book The Complete Beatles Recording Sessions, says the song included "distorted, hypnotic drum and organ sounds, a distorted lead guitar, the sound of a church organ, various effects (water gargling was one) and, perhaps most intimidating of all, John Lennon and McCartney screaming dementedly and bawling aloud random phrases like 'Are you alright?' and 'Barcelona!'" McCartney biographer Barry Miles wrote in Paul McCartney: Many Years from Now that the song had "no rhythm, although a beat is sometimes established for a few bars by the percussion or a rhythmic pounding piano. There is no melody, although snatches of a tune sometimes threaten to break through." ...Also, according to Barry Miles, musically it "resembles 'The Return of the Son of Monster Magnet' from Frank Zappa's Freak Out! album, except there is no rhythm and the music ... is more fragmented, abstract and serious."
Link to "Return of the Son of Monster Magnet" - that Carnival of Light is supposed to sound similar to
sounds like John Lennon screaming at 00:22 Imagine what Carnival of Light must have sounded like though, when it was first heard, with those kind of sounds, and a light show going on at the same time... must have been the ultimate psychedelic experience... More from Wikipedia link above:
In November 2008, McCartney confirmed he still owned the master tapes, adding that he suspected "the time has come for it to get its moment. I like it because it's the Beatles free, going off-piste." McCartney would need the consent of Lennon's widow, Yoko Ono, and George Harrison's widow, Olivia Harrison, as well as Ringo Starr to release the track. In 2016, McCartney stated in an interview with Rolling Stone he was toying with the idea of releasing unused Beatles' recording takes. During this interview he again admitted he was still toying with the idea of releasing Carnival of Light. As of 2017, the track remains unreleased. Many anticipated it to be included as a bonus track for the upcoming super deluxe edition of Sgt. Pepper's Lonely Hearts Club Band, but it failed to appear in the released track listings. At a preview event hosted at Abbey Road Studios to unveil the new Pepper stereo remix, Giles Martin commented that while it was considered at one point for inclusion, "it wasn't really part of Pepper. It wasn't part of the Sgt. Pepper recording. It's a very different thing". However, he expressed an interest in "do[ing] something interesting" with the track in the future.