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A hidden message is information that is not immediately noticeable, and that must be discovered or uncovered and interpreted before it can be known. Hidden messages include backwards audio messages, hidden visual messages and symbolic or cryptic codes such as a crossword or cipher.
Although there are many legitimate examples of hidden messages, many so-called hidden messages are merely the fanciful imaginings of conspiracy theorists.
Backward audio messages 
A backward message in an audio recording is only fully apparent when the recording is played reversed. Some backward messages are produced by deliberate backmasking, while others are simply phonetic reversals resulting from random combinations of words.
Backmasking is a recording technique in which a message is recorded backwards onto a track that is meant to be played forwards. It was popularized by The Beatles, who used backward vocals and instrumentation on their 1966 album Revolvere technique has also been used to censor words or phrases for "clean" releases of songs.
Backmasking has been a controversial topic in the United States since the 1980s, when allegations of its use for Satanic purposes were made against prominent rock musicians, leading to record-burnings and proposed anti-backmasking legislation by state and federal governments. In debate are both the existence of backmasked Satanic messages and their purported ability to subliminally affect listeners.
Phonetic reversal 
Certain phrases produce a different phrase when their phonemes are reversed—a process known as phonetic reversal. For example, "kiss" backwards sounds like "sick," and so the title of Yoko Ono's "Kiss Kiss Kiss" sounds like "Sick Sick Sick" or "Six Six Six" backwards. The Paul is dead phenomenon was started in part because a phonetic reversal of "Number nine" (the words were constantly repeated in Revolution 9) was interpreted as "Turn me on, dead man".
"Weird Al" Yankovic uses this technique in two different songs. The first was "Nature Trail To Hell" in which he states "Satan Eats Cheese Whiz". The next song to have a backwards message was "I Remember Larry" in which he states "Wow, you must have an awful lot of free time on your hands!".
According to proponents of reverse speech, phonetic reversal occurs unknowingly during normal speech.
In the computer game Doom II, a garbled message played at the start of Map 30, spoken by the "Icon of Sin", can be played backwards to hear "To win the game, you must kill me, John Romero." Romero was a programmer for the game; he put the backwards message (with distortions) in to get back at the artists who put the image of his head on the final level.
Stanley Kubrick, in his 1999 release, Eyes Wide Shut, features a scene depicting a Satanic ritual, where the soundtrack (the song "Masked Ball") consists of a backwards Orthodox liturgy chanted in Romanian.
In one scene of Beavis and Butt-Head Do America, Beavis and Butt-Head hallucinate, and voices are heard in the background. The voices are the two characters speaking phrases such as "Everybody go to college, study hard, study hard."
In the Clone High episode "Raisin the Stakes", JFK falls through the cafeteria sunroof, lands, and begins to foam at the mouth and speak gibberish. The gibberish played backwards is JFK saying "I am talking backwards, and telling you to watch Clone High ... and for us to get an Emmy ... I'm saying that backwards ... 'cause it's sneaky!"
The episode "Expiration Day", from The Brak Show, includes the message "Why are you listening to this song backwards, you could have been on a date with a girl". It is sung by Thundercleese as he was going to be shut down.
In the SpongeBob SquarePants episode "Opposite Day", one of the "Opposite" thing SpongeBob and Patrick do is speak gibberish. When played backwards, it goes:
- SpongeBob and Patrick laughing
- SpongeBob: To get to the opposite side!
- Patrick: I give up.
- SpongeBob: Hi Patrick.
This message seems to be based on the saying Why did the chicken cross the road?
In Episode 130 of the hugely popular anime series Pokémon (The Whistle Stop), James accidentally starts to get eaten by one of his own Pokemon and speaks gibberish when it happens. The gibberish played backwards, says "Leo Burnett and 4Kids are the Devil! Leo Burnett!".
During the ending theme to Cartoon Network's The Grim Adventures of Billy and Mandy, some strange, somewhat creepy gibberish is heard at the end. The gibberish played backwards is Maxwell Atoms, the creator of the show, saying "No, no. This is the end of the show. You're watching it backwards!"
Visual messages 
In the video game Mercenary (1985) one item to pick is "Essential 12939 supply", a box with numbers 12939. When walked around numbers are seen as letters "PEPSI".
In the British game show Robot Wars, one of the robots that competed in Series 5 and 7 was a robot called 8645T. When looked a certain way, it would read BEAST.
In the 1980s, Coca-Cola released in South Australia an advertising poster featuring the reintroduced contour bottle, with a speech bubble, "FEEL THE CURVES!". Inside one of the ice cubes was the silhouette of a woman performing fellatio. Thousands of posters were distributed to hotels and bottle shops in Australia before the mistake was discovered by Coca-Cola management. The artist of the poster was fired and all the posters were recalled.
Various other messages have been claimed to exist in Disney movies, some of them risque, such as the well-known allegation of an erection showing on a priest in The Little Mermaid. According to the Snopes website however, only one "is clearly true [and] undeniably purposely inserted into the movie": images of a topless woman in two frames of The Rescuers.
PETA (People for the Ethical Treatment of Animals) is also the scene of a running battle with PETCO, a pet food retailer in San Diego, for several years regarding the purported mistreatment of live animals at PETCO stores. When the San Diego Padres announced that PETCO (which is based in San Diego) had obtained naming rights to PETCO Park, PETA was unable to persuade them to terminate the agreement. Hence PETA used advertising in the form of an acrostic. It successfully purchased a brick with what appears on the surface to be a complimentary message: "Break Open Your Cold Ones! Toast The Padres! Enjoy This Championship Organization!" However, if one takes the first letters of each word, the resulting acrostic reads "BOYCOTT PETCO". Neither PETCO nor the Padres have taken any action to remove the brick, stating that if someone walked by, they would not know it had anything to do with the PETA/PETCO feud.
- "PC Interview: John Romero". Computer and Video Games. Retrieved 2006-08-19.
- The Kubrick Site: Introducing Sociology by Tim Kreider
- Beavis and Butt-Head Do America (1996) - Trivia
- "Clone High" (2002) - Trivia
- "Lost Character Walt Lloyd". Retrieved 2007-02-03.
- "GI Joe: Destro Cult Message". The Easter Egg Archive. Retrieved 2006-08-27.
- "The Brak Show: Expiration Day". TV.com. Retrieved 2006-07-25.
- "Risque Coca-Cola Poster". Snopes. 14 May 2011. Retrieved 31 May 2011.
- Urban Legends Reference Pages: Disney (Disney Films)
- Rovell, Darren "Secret Message Makes it Into New Park" ESPN.com 4/16/2004
- Audio Reversal in Popular Culture — explanation of backmasking and phonetic reversals