1. "Cerberus and the Three-Headed Dog":
>When Percy, Annabeth, and Grover encounter Cerberus guarding the entrance to the Underworld, it alludes to the mythological multi-headed hound responsible for protecting the gates of Hades according to Greek mythology.
2. "The River Styx and Charon the Boatman":
>As the trio cross the River Styx on Charon's boat toward the Underworld, this event parallels how Charon guides souls in Greek mythology across the same river into the afterlife world.
3. "The Garden of Hesperides and Ladon the Dragon":
>Percy describes an encounter they had where they fought "Ladon, this huge-dragon thing...coiled up around tree like a Slinky." This references another famous mythological creature and story about the Garden of Hesperides guarded by the many-headed dragon Ladon in Greek myths.