Mount Diablo is a mountain in Contra Costa County in the San Francisco Bay Area (3,849 feet).
Mount Diablo is where the goddess Athena trapped Enceladus a millennia ago after the first Gigantomachy. Gaea freed him and Enceladus now uses the mountain to imprison his enemies like Tristan McLean. After being told the location by Aeolus, Jason Grace, Piper McLean, Leo Valdez, and Coach Hedge went to Mount Diablo to free Piper's dad. While the were climbing up the mountain, Gaea attempted to stop them by making the path like quicksand. Once, they reached the top, the Gigas sent Earthborn to attack them, they were defeated by Leo and Piper. Jason killed Enceladus with the help of his father Zeus and re-trapped him in the mountain.
After Meg McCaffrey is placed on the Throne of Memory, she recites a prophecy of the Oracle of Trophonius. The second line is Ere new moon rises o'er the Devil's Mount, which makes Leo believe it refers to Mount Diablo.
- The Native Americans of the area, Chumash, lost a battle against the Spaniards there earning the mountain its name, the Devil Mountain (in Spanish). It is considered a cursed yet holy place for them.