I've read accounts that state clearly that naval battles were held in the Colosseum, but I also know that there were substructures beneath the floor of the arena that allowed for the dramatic entrance of animals, people, and stage sets via trap doors. Can anyone enlighten me how it's possible that the building was watertight enough to actually flood and stage naval battles? I'm feeling that these stories are exaggerated or apocryphal and perhaps the naval battles were just simulated.