The Colosseum, could not have been built without concrete technology. The enormous oval seating area is sustained by a complex system of radial and concentric corridors covered by concrete barrel vaults. This concrete "skeleton" reveals itself today to anyone who enters the amphitheater; in the centuries following the fall of Rome the Colosseum served as a convenient for ready-made building materials, and almost all its marble seats were hauled away, exposing the network of vaults below.