There is a closed area under the amphitheater floor called the Cruciform Room. These are the remains of the ancient underground backstage area.
The ancient walls: The walls of the two tunnels that intersect are ancient walls. This backstage or understage area was built just after the amphitheater itself.
The modern walls: The tunnels end in modern walls built in the 19th century when the ruins were discovered.
The understage area included: - two large perpendicular central tunnels (21 feet and 19 feet wide) - a peripheral tunnel (about 6.5 feet wide) - four other tunnels accessible from the peripheral tunnel and the tunnel along the long axis.
The understage areas could be accessed through tunnels located below the two end arches of the monument’s long axis.