Tartarus, TAHR tuhr uhs, was a deep pit below the surface of the earth in early Greek mythology. It was as far below the surface as heaven was above the earth. High walls and a river of fire called Phlegethon (or Pyriphlegethon) encircled Tartarus. Zeus, ruler of the gods, imprisoned the rebellious gods called Titans there. Any god who swore a false oath by the River Styx in the Underworld was kept in Tartarus for nine years (see Styx).