The Hypogeum is the Colosseum's substructure. It was constructed around ten years after Domitian's opening of the Colosseum. The Colosseum was erected later because it had already held naval combat. The construction was made of masonry and had a network of tunnels with two principal pathways. One led to private houses while another one connected the stadium to the Roman Forum. The Hypogeum was used for various purposes including storage, which included food for the games as well as supplies for the spectators. In addition, it provided accommodation for trainers and officials who attended the games. Last but not least, the hypogeum served as a treasury where money collected from people visiting the Colosseum was kept. Over the years, parts of the hypogeum have been closed off to make more room for shops and restaurants. However, it is still possible to see some portions of it with its original features such as columns, statues and arches intact. If you go to the Colosseum today, you will see two main levels: the ground floor and the first floor. The first floor contains offices, dressing rooms, toilets, and other facilities needed by the employees working at the Colosseum. The ground floor has three entrances: one for visitors, one for staff members, and one for athletes.