Not all cryptograms are easy to solve. In fact, there are many famous unsolved codes and ciphers. The Voynich manuscript, which is over 400 years old, is a nearly 235-page manuscript that is written entirely in a strange, unknown alphabet.
Another unsolved cipher was one sent on July 14, 1897, by composer Edward Elgar. He sent a letter to a friend, Dora Penny, written completely in code. She was unable to read the letter, and he never explained it. Miss Penny attempted to have other people solve it, but to this day the code has not been cracked.
In another example of an unsolved code, archaeologist Sir Arthur Evans uncovered clay tablets with mysterious symbols in Crete in 1900. He spent a great amount of time attempting to decrypt the three different writing systems used (a hieroglyphic script, and two systems later referred to as Linear A and Linear B). To this day, only Linear B has been solved.
In the late 1960s, a serial killer, nicknamed the Zodiac Killer, sent multiple coded communications to the police and editors of publications. Some of those letters were solved, but many were not.
The Edgar Allan Poe Cryptographic Challenge is a famous cryptogram that was just recently solved. Poe was enthralled with cryptography and challenged his readers to send him cryptograms to solve. A “Mr. W. B. Tyler” submitted two cryptograms to Poe and Poe never found solutions for them. Instead he challenged his audience to solve them. It was speculated that perhaps Poe created the cryptograms himself, but nobody has been able to prove it. Gil Broza of Toronto cracked the cipher 150 years later.