If you have watched the movie, Gladiator, you wil remember other gladiators and a tiger appearing from below the arena.
I have to admit I thought that was a bit farfetched.
I now understand that it was the case.
Looking at the actual Colosseum you can see the maze of corridors and pens beneath the area itself.
An awesome place.
Photo: M Muir - Inside the Roman Colosseum - The area at the far end where the floor level has been recreated gives an indication of the maze below the area where gladiators and wild animals were housed.