There aren't too many songs Beatles fans are dying to hear. When it comes to lost songs by the group, pretty much everything they've recorded is out there on official albums or on bootlegs.

But there's one track recorded during the Sgt. Pepper's Lonely Hearts Club Band sessions that's a holy grail of sorts for fans: an experimental piece from early 1967 called "Carnival of Light," which can be thought of as a precursor to the White Album's "Revolution 9." (There were rumors that the song would finally get an official release on Sgt. Pepper's expanded reissue, but it didn't show up there.)

The song was created for two events that were being held on Jan. 28 and Feb. 4 at London's Roundhouse Theatre that was alternately known as both the Million Volt Light and Sound Rave or the Carnival of Light Rave. David Vaughan, who was promoting the happenings, asked Paul McCartney to contribute some music, and the Beatle agreed. On Jan. 5, 1967, during a break in the sessions for "Penny Lane," they created "Carnival of Light."

In November 2008, McCartney told the BBC what his instructions to the other three Beatles were. "I said all I want you to do is just wander around all the stuff, bang it, shout, play it, it doesn't need to make any sense," he said. "Hit a drum then wander on to the piano, hit a few notes, just wander around. So that's what we did and then put a bit of an echo on it. It's very free."

The track runs 13:48, according to Mark Lewisohn's The Beatles Recording Sessions: The Official Abbey Road Studio Session Notes, 1962-1970, making it the longest uninterrupted recording the Beatles had done to date. "Track one of the tape was full of distorted, hypnotic drum and organ sounds," Lewisohn writes. "Track two had a distorted lead guitar; track three had the sounds of a church organ, various effects (the gargling with water was one) and voices; track four featured various indescribable sound effects with heaps of tape echo and manic tambourine."

Lewisohn describes the voices as being McCartney and John Lennon screaming random phrases like "Barcelona!" and "Are you all right?"

It all ended abruptly when McCartney shouted, "Can we hear it back now?" A rough mono mix was then made, and McCartney took it to Vaughan. Engineer Geoff Emerick recalled producer George Martin as saying, "This is ridiculous. We've got to get our teeth into something a little more constructive."

Beatles Bible says that McCartney had hoped to include "Carnival of Light" on Anthology 2, but George Harrison was against it.

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