Zodiac 340 Cipher
The notorious Zodiac Killer was infamous for sending coded letters to the police and news agencies. In 1969, he sent a cipher containing 340 symbols which have never been decoded. The letter was analyzed by many experts including the FBI, however, no one was successful in finding out the Zodiac Killer's message. The 340 cipher is among the most famous secret codes in the world and some programmers have even developed a website named "Zodiac Web Toy" where you can try to crack the letter.
Isaac and the Alien Cipher
A man called Isaac allegedly claimed that he was an employee of a secret government laboratory in Palo Alto, California. According to his story, in the 1980s, Isaac was working on alien tech and he left behind a series of alien ciphers. His claims were investigated for many years but experts have never been able to find the secret facility or crack the extraterrestrial messages. Isaac shared five strange diagrams which featured obscure scripts, symbols, and captions.