The Colosseum (the Flavian Amphitheater), begun under the emperor Vespasian around AD 72 and completed by his son Titus in AD 80, was the largest amphitheater ever built in the Roman world, seating an estimated 50,000-80,000 spectators. Built of concrete and stone, it hosted gladiatorial combats, staged animal hunts (venationes), and, in its early years, even mock naval battles (naumachiae), for which the arena floor could reportedly be flooded. Beneath the arena floor lay the hypogeum, a complex network of tunnels and chambers where gladiators, animals, and stage machinery were held before being raised into the arena by an elaborate system of lifts and pulleys. The building’s exterior facade, with its stacked tiers of arches and classical columns (Doric, Ionic, and Corinthian), became a model imitated in countless later structures.