It's possible you're thinking of:
* The Tartarus Abyss: This is a massive chasm within the Underworld, mentioned in *The Titan's Curse* and *The Battle of the Labyrinth*. It's a terrifying place, and Percy travels through it in *The Battle of the Labyrinth* with Annabeth.
* The Pit of Tartarus: This is a pit within the Underworld, and is where the Titans are imprisoned. It's also described in *The Titan's Curse* and *The Battle of the Labyrinth*.
Are you sure you're thinking of *The Lightning Thief*? If you can give me more context, I can help you figure out what you're remembering!