Cristiano Ranieri pulls a dry suit over his head and fixes a full-face respirator in place. Above, the hum of tourist traffic bounces off the travertine and brick surfaces of the Colosseum. But down here, among the maze of passages where gladiators would have waited and lifts would have raised lions, bears, and other exotic animals up to the action, the sound is muffled. Ranieri becomes visibly excited as he describes scuba diving beneath the Colosseum, explaining that this space, inside the 40-foot (12-meter) deep "doughnut" foundation that holds up the rest of the structure, isn't even the bottom. He removes a steel plate from the floor to reveal a still body of dark gray water several yards below: the underbelly of the underbelly of the Colosseum.
Ranieri is scouting a new access point into the drain system for a future measuring project. In particular, he wants to know whether he will be able to carry a full underwater lighting rig into this hole or be forced to use portable flashlights. A well-lit swim below the Colosseum could change history.
Until three years ago only a quarter of the conduits—the driest and most easily accessible—below the Colosseum had been explored. These simple drains, designed to whisk away storm water, date from the late first century a.d., when the Flavian emperors were building the Colosseum. Some ancient writers claimed the building was deliberately flooded for mock naval battles. But there was no evidence of the large waterworks needed to bring in the water.
Then, in October 2003, Ranieri, an archaeologist and speleologist with the superintendency of archaeology, made a startling discovery. Below the simple drains (and predating the Colosseum) were large conduits constructed by Emperor Nero to charge an artificial lake in his gardens. The conduits had obviously been reused by the architects of the Colosseum, most likely to pipe quantities of water in and out. For the first few years of its history, at least, the Colosseum, like many other theaters, was capable of being flooded.
Far more common than planned expeditions to reveal Rome's hidden secrets are the chance discoveries: A work crew digs a hole in the street and cracks into a hollow underground space. Speleologists are called in, and yet another astounding find alters the picture. Such was the case on the Oppian Hill two winters ago when, after a period of intense rain, a hole spontaneously opened near a tree, exposing a matrix of underground rooms.