Darius I, also known as Darius the Great, had inscriptions carved on a massive rock face at Behistun in modern-day Iran. These inscriptions told the story of his rise to power and were written in three different languages: Old Persian, Elamite, and Akkadian (Babylonian).
The key to deciphering cuneiform was recognizing that the same message was written in three different languages. This allowed scholars to compare the different versions and eventually piece together the meaning of the cuneiform symbols.