The Great Trapezium is one of the most extraordinary and least known structures of Villa Adriana, and it has never been open to the public. It has four large tunnels 350, 300, 125 and 100 meters long, and almost 6 meters high and wide.
They were hand-dug into the tuff rock located south of the Accademia, providing building material for the Villa. On the walls you can still see millions of signs of the pickaxes of the slaves who excavated them.
One of the tunnels of the Great Traezium with the oculus for lighting in the vault
In the 16th century the Great Trapezium was identified with the "Underworld" (Inferi) of Villa Adriana mentioned in the Historia Augusta. In the 19th century, Giuseppe de’ Bardi described it as: «full of infernal mouths, which threw fire to others' liking». There, of course, «lived Cerberus, who, to his talent, made Hell boom».
About every ten meters in the vault of the galleries large circular oculi opened; they are two meters in diameter and illuminated he galleries. There are 79 oculi, and by following them from the outside it was always possible to reconstruct the path of the galleries, without venturing inside them.
The Great Trapezium was a real underground 'subway', which served to hide from sight and hearing the traffic of the carts bringing supplies to the Villa, where a very large Court lived. They brought food, wine, oil for the lamps, various furnishings and even the wood that was used for heating water in the thermal plants.
In the four tunnels of the Great Trapezium two carts can pass in both directions. Then there are other narrower tunnels, where only one cart passed at a time; they were connected to a extensive network of subterranean pedestrian corridors, meant for the slaves, which reached the various buildings of the Villa.
This subterranean road network was similar to what we see in modern cruise ships, where a network of hidden and parallel service routes exists alongside the luxurious quarters for the passengers.