The ancient structure that served as a mausoleum for the Persian king Darius I is known as the Tomb of Darius, located at Naqsh-e Rustam in modern-day Iran. Darius I, also known as Darius the Great, ruled from 522 to 486 BC and is renowned for his administrative achievements and expansion of the Persian Empire. The tomb features impressive rock-cut architecture and is adorned with intricate reliefs depicting Darius and his royal lineage. It stands as a testament to the architectural sophistication of the Achaemenid Empire and exemplifies the enduring legacy of Darius I as a significant historical figure.