Continuity mistake: At the start of Phil's song, he is standing on a pillar. The angle of this pillar varies between shots.
Continuity mistake: When Pain and Panic are shown for the first time, the cracks on the ground change between shots.
Continuity mistake: When Hercules comes to the underworld, he sees Meg floating in the river next to a lot of other souls. Then as Hercules reaches down to get her, hardly any souls are floating next to her. (01:11:25)
Continuity mistake: Hercules has his ribbon around his neck in the temple. As he's praying, a wind suddenly blows up. The ribbon around his neck is gone when this happens.
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