Codes have done everything from transmitting crucial information during World War II to keeping little brother from snooping in your diary. Here are some that have remained uncrackable.
The Zodiac Killer's codes
It's been 40 years since the serial killer terrorized the San Francisco Bay Area. The murderer sent letters to the newspapers and police, and four contained coded messages. Only one was cracked. Mental Floss notes that the 408-character code -- deciphered by Donald and Bettye Harden -- contained mistakes and spelling errors, possibly making the other three messages so difficult to untangle.
But amateur cryptanalysts still try to break the codes: Last year Corey Starliper, a Zodiac-killer obsessive, said he figured out the toughest one, even though police and experts dismissed his claim. If you want a challenge, check out this Web tool that allows you to take a crack at the cipher yourself.
A simple sequence of letters is all it is: O U O S V A V V is carved between the letters D and M on the 18th-century Shepherd's Monument located at Shugborough Hall in Staffordshire, England. The monument was commissioned by Thomas and Admiral George Anson, and the letters have never been fully explained, therefore having the dubious honor of being one of the great unsolved cipher texts.
Theories abound. The inscription has been thought to be a love letter to Anson's wife, an abbreviated Latin phrase, and even a link to the Priory of Sion -- a code that would lead to the Holy Grail. Dan Brown's novel, "The Da Vinci Code," had a similar idea, prompting new interest in the inscription. But so far, nobody has cracked it, not even Tom Hanks.
In 1990, a sculpture was created for the CIA that incorporated four encoded messages as a challenge to the employees at the agency. While people at various organizations, including the CIA and NSA, have said they have solved three of them, the fourth remains the toughest to decipher, since it contains only 97 or 98 characters.
Richard Feynman's challenge ciphers
In 1987 physics professor Richard Feynman received three encoded messages from a fellow scientist at Los Alamos, California, and shared them with his graduate students. Only one of the three has ever been solved. It turned out to be the opening lines of Chaucer's "Canterbury Tales" written in Middle English. The unsolved puzzles are posted here.
Check out the junior code cracker in "Electric City,"the new postapocalyptic series from Tom Hanks, and follow @ElectricCityAMP for more clues.
And catch up with the series by watching the first episode.