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View of the underground of the Amphitheater at Puteoli (modern Pozzuoli). This is one of two concentric ellipses that, along with a long straight central corridor, that form the expansive area where scenery and animals were kept. Note the brick over rubble columns and the concrete barrel vaulting of the ceiling that in turn supported the (I believe concrete covered with sand) floor of the arena above.
Columns from above were dumped here over time as the upper area was used for gardens and houses and even a church.
This amphitheater was constructed in the late first century A.D. and is called the "Flavian Amphitheater." It was probably built about the same time as the Colosseum in Rome and is the third largest amphitheater in Italy. It could hold up to 50,000 spectators.