Artificial lake by Sharrie
Realise that The Colosseum was built on the site of an artificial lake around which NERO's royal residence was centered. The purpose? Emperor Vespasian wanted to restore to the Roman people what Nero had tyrannically deprived them of & to provide Rome with a large amphitheater.
Pictured you see the stretches of cavea with connecting corridors & the vaulted passageways inside.