Continuity mistake: After the cyclops has beaten Hercules up, Phil arrives to talk some sense into him and drops his handkerchief on the ground. Then when the cyclops picks Hercules up, the handkerchief is gone.
Continuity mistake: Hercules meets Phil. Phil walks away & enters his tree. Hercules grabs the door & yanks it off it's hinges. You can see there's a hinge on the door & another was left in the doorway's frame. Phil is hanging onto the door handle as Herc holds it up. After Phil lets go of the door, the door has both hinges.
Continuity mistake: When the pillar almost falls on Hercules, Meg dives head first to push him away. Then in the next shot, as Meg is crushed under the pillar, she's lying feet first.
Continuity mistake: When Hercules is running over the log, the stick he trips over becomes longer between shots.
Continuity mistake: When Hercules catches the Frisbee, he falls on the ground and makes several cracks. Seconds later, the cracks are gone.
Continuity mistake: After Hercules defeats the hydra, the pile of rocks that crushed the hydra changes between shots.
Continuity mistake: In the stadium, Hercules twirls Phil around and forms a hole. The shape of this hole changes between shots.
Continuity mistake: The size of the Titans constantly changes throughout the scenes involving them. This is particularly true for the Rock Titan.
Continuity mistake: When Hades throws the steak to Cerberus, the size and shape of the steak changes noticeably between shots.
Continuity mistake: When Hercules runs into the market, he stops himself and makes a hole in the ground. The length of this hole changes quite distinctively between shots.
Continuity mistake: During the battle with the hydra, the big rock that Hercules throws at it appears out of nowhere.
Continuity mistake: After Hercules makes the village fall down the potmaker smashes a pot in front of Hercules. A few moments later in an overhead shot the smashed pot is gone.
Continuity mistake: When the blonde kid takes the Frisbee away, saying "Nice catch Jerkules", the yellow circle that was in the middle of the circle earlier is gone.
Continuity mistake: After Zeus tells a joke, he falls into his chair saying, "I kill myself," but his lips don't match what he's saying.
Factual error: When Hercules and Phil are flying over Thebes, the ocean is visible nearby. Thebes is nowhere near the ocean.
Continuity mistake: When Hercules is showing Zeus how he beat the gorgon, he pushes Pegasus into a nearby pool of water. This pool was not there the first time Hercules came to the temple.
Continuity mistake: Pegasus flies into the temple of Zeus through an opening in the ceiling, then when he and Hercules are flying over it the opening is gone.
Continuity mistake: During the song "Zero to Hero" a large golden statue of Hercules is unveiled in Thebes. Later in the film, when the cyclops is attacking the city, the statue is in the same location, but it is much smaller, and now it is made of stone.
Continuity mistake: Before Meg's song, the tree Hercules takes a flower from appears out of nowhere.
Continuity mistake: When the scissors break after the Fates fail to cut Hercules' string of life, the way the scissors are bent changes between shots.
Answer: I always assumed one of the gods told him how to get there after they won the battle. The underworld in Greek mythology could be traveled to by mortals, albeit extremely ill-advised.
Jordan Oxendine