Probably at first — but how is a mystery
Roman writers like Martial and Cassius Dio describe a mock naval battle (naumachia) at the Colosseum's inauguration under Titus, so a flooded-arena spectacle likely did happen early on. But no Roman author recorded how it was engineered, and almost nothing survives in the archaeology, so the exact method stays debated. The combatants were not star gladiators: both sides were condemned criminals and prisoners of war, fighting what was essentially a staged mass execution with few survivors. Flooding was only feasible before the underground hypogeum was built beneath the floor.
