The underground part of Rome's Colosseum contains passages, rooms, and transport facilities that supported the entrance of animals and people. The exposed underground structures seen today were not the original spectators' view.
Opened in the 1st century CE, the building used tiers of arches and seating. It hosted public spectacles, and its later uses and state of preservation continued to change.
Arena and hypogeum
The arena itself was 83 meters by 48 meters (272 ft by 157 ft / 280 by 163 Roman feet). It comprised a wooden floor covered by sand (the Latin word for sand is harena or arena), covering an elaborate underground structure called the hypogeum (literally meaning "underground"). The hypogeum was not part of the original construction but was ordered to be built by Emperor Domitian. Little now remains of the original arena floor, but the hypogeum is still clearly visible. It consisted of a two-level subterranean network of tunnels and cages beneath the arena where gladiators and animals were held before contests began. Eighty vertical shafts provided instant access to the arena for caged animals and scenery pieces concealed underneath; larger hinged platforms, called hegmata, provided access for elephants and the like. It was restructured on numerous occasions; at least twelve different phases of construction can be seen.
The hypogeum was connected by tunnels to a number of points outside the Colosseum. Animals and performers were brought through the tunnel from nearby stables, with the gladiators' barracks at the Ludus Magnus to the east also being connected by tunnels. Separate tunnels were provided for the Emperor and the Vestal Virgins to permit them to enter and exit the Colosseum without needing to pass through the crowds.
Substantial quantities of machinery also existed in the hypogeum. Elevators and pulleys raised and lowered scenery and props, as well as lifting caged animals to the surface for release. There is evidence for the existence of major hydraulic mechanisms and according to ancient accounts, it was possible to flood the arena rapidly, presumably via a connection to a nearby aqueduct. However, the construction of the hypogeum at Domitian's behest put an end to the practise of flooding, and thus also to naval battles, early in the Colosseum's existence. However, there is ample debate over the practicality and logistics of this phenomenon, which leads to speculation over its actuality. This is mostly because there is no physical evidence of naumachiae at the Colosseum remains.
Construction, inauguration, and Roman renovations
The site chosen was a flat area on the floor of a low valley between the Caelian, Esquiline and Palatine Hills, through which a canalised stream ran as well as an artificial lake/marsh. By the 2nd century BC the area was densely inhabited. It was devastated by the Great Fire of Rome in 64 AD, following which Nero seized much of the area to add to his personal domain. He built the grandiose Domus Aurea on the site, in front of which he created an artificial lake surrounded by pavilions, gardens and porticoes. The existing Aqua Claudia aqueduct was extended to supply water to the area and the gigantic bronze Colossus of Nero was set up nearby at the entrance to the Domus Aurea.
Although the Colossus was preserved, much of the Domus Aurea was torn down. The lake was filled in and the land reused as the location for the new Flavian Amphitheatre. Gladiatorial schools and other support buildings were constructed nearby within the former grounds of the Domus Aurea. Vespasian's decision to build the Colosseum on the site of Nero's lake can be seen as a populist gesture of returning to the people an area of the city which Nero had appropriated for his own use. In contrast to many other amphitheatres, which were on the outskirts of a city, the Colosseum was constructed in the city centre, in effect, placing it both symbolically and geographically at the heart of Rome.