The realm of the dead, ruled by Hades. In Riordan's version, the Underworld is accessed through various entrances including a recording studio in Los Angeles. It contains the Fields of Asphodel (where ordinary souls wander), Elysium (paradise for heroes), the Fields of Punishment (for the wicked), and Tartarus (the deepest, darkest pit, essentially a primordial entity as much as a place). The River Styx flows through it, and bathing in its waters grants invulnerability.