S V B S T R U C T I O
This self-explanatory image is taken from C.F. Giuliani "Ledilizia nellantichità"
The original site
was deeply transformed in order to build the amphitheatre. First of all, some enormous drains were built in order to ensure an adequate drainage towards the Circus Maximus (a part of these drains was lost when the metro line was built).
After the area was completely drained, the excavation started, and it lasted until it reached the clay bed of the lake. In the firm clay bed an elliptical ring was excavated, 31 metres wide, 6 metres deep, with a perimeter of 530 metres. This enormous excavation was filled up with Roman cement, i.e. mortar made with pozzuolana and lime, mixed with coarse crushed stones. Layers and layers of mortar and stones were laid, and the concrete was compacted by hammering. It seems that on the SW side the clay bed wasn't as firm as on other sides, and this could be the reason why that side collapsed first.
Then the foundation was raised for a further 6 metres, so that the thickness of this enormous doughnut is over 12 metres. All around the foundations a reinforcement brick wall was built, 3 metres wide and 6 metres deep, and a similar wall was built inside. On the internal brick wall were arranged 32 cells, that are visible all around the underground of the arena.
Drillings recently carried out have shown that, contrary to what Ingegner Cozzo thought (and to what is still widely believed) there is no underground storey of travertine arches beneath the ground floor. The only foundation is this formidable ring of cement, and the only underground arches at the bottom of the arena are the ones on the main axis. The drillings have demonstrated that the top of the foundation reaches up just beneath the ground floor. The foundation can be also seen from the so-called Passage of Commodus, a fifth tunnel decorated with stuccoes which was excavated after the Colosseum was completed.
All the surrounding buildings were demolished (probably also including many buildings of the Domus Aurea) and with these materials and earth, the valley was filled up to the level we see today. It is believed that the only thing surviving from the former arrangement was the meta sudans (the sweating post), an ancient fountain that was placed at a crossroad where the borders of four traditional boroughs of the city met.
In the foundations and in the external wall along the axis - there are the four underground tunnels and below them four big drains (1.3 by 3.8 metres). These passages were made during the building of the foundations, by casting the concrete around a wooden boxing. Some remains of the boxing, which were made of non-seasoned oak timber, have been dated back - quite obviously - to about the year 70 AD. More large underground rooms, necessary for the services and the preparation of the shows, were made along the main axis.
Four plus one underground tunnels connected the amphitheatre to the surrounding buildings: the NE passage, under the main axis (Porta Libitinaria), reached the Ludus Magnus, it was cleared by the French for 83 metres, then filled up again and it was interrupted by a modern drain in the 19th century. The one on the opposite end led to the Summum Choragium, where the scenarios and costumes of the shows were prepared before the place was occupied by the temple of Venus, and it was sliced by the works for the underground railway in the '40s. On the short axis, the eastern tunnel towards the Exquiline Hill, 2.95 metres wide, and the other one towards the Celium seems, quite surprisingly, haven't been explored yet. Near the tunnel on the side of the Celian Hill,a fifth passage, called "Passaggio di Commodo", is believed to connect the amphitheatre and the imperial palace on the side of the Celian Hill. It owes its name to the emperor Commodus, who had it excavated. There are remains of beautiful decorations with frescoes, mosaic and marbles and it had skylights to receive illumination. This passage, too, has never been explored completely.
The cubicles built all around the bottom of the arena were, according to many, used to keep the wild beasts during the shows. Cozzo actually thought that the beasts were restricted to the narrow underground corridor along the sides of the bottom of the arena, so that they were obliged to reach their respective cages by a system of gates. Moreover, with the animals confined along the sides, the central corridors could be free for transit.
For a page on the visit to the underground spaces click here.