The big building was built between the second and first centuries BC, when Amelia was elevated to the rank of municipality and equipped with a series of functional infrastructure (walls, terracing, roads).
It consists of a large rectangular room (57,50 x 19,60 meters) divided into ten parallel compartments covered with barrel vaults (highest on average 5.70 meters) carved into the limestone rock and then lined by masonry, it has a cement core and a vestment of rough stones set in the core. The state of conservation is exceptional, with all the fundamental components still in place for the operation of the entire complex, including the water supply system, the internal device for regulating the maximum water level and the tank emptying system.