How an underground labyrinth threatens Rome's monuments

NARRATOR: Rome is special and unique - a legend that continues to live today. But the city's antique heritage is endangered. The Vittoriano isn't the only monument threatened. Fissures criss cross the stones, Rome's landmarks are sinking. The city must repeatedly undertake measures to shore up the monuments. Rome is constructing a new metro line, Michele Jamiolkovski fears the worst.

MICHELE JAMIOLKOVSKI: "We expect the monument will sink at least 10 millimeters, we have to do something to avoid major damage."

NARRATOR: City planners have a hunch the city's underbelly is causing the buildings to sink. A team of explorers feels its way along the tuffaceous arches of the Cloaca Maxima, which has been part of Rome's sewage system for 2,500 years. Only a few meters under the city a parallel world exists, and no one knows just how extensive it is. The explorers intend to survey this labyrinth with a GPS and a compass. Their aim: to map it as precisely as possible.

Unknown cavities pose great danger to the monumental structures above. The damage that would be caused if they were to collapse would be unimaginable. Treasures like these are hidden under the Romans' feet, too. As the underground researchers forge ahead, engineers try their luck above ground. They use electrodes to send electrical charges into the ground. The conductivity tells quite a bit about the composition of the earth below. Cavities are easily detected, as air is a poor conductor. They continue to find systems of caverns. Rome's underbelly is like Swiss cheese.

And today, once again, they make a surprising discovery - a soil sample. It's puzzolan. Ancient Romans used this stone to make cement. The material that holds Rome together, even today. They take a video camera with them to explore the drill hole more carefully. Three meters down everything changes, the drill hole widens. The camera peers into complete nothingness. What does this mean? Experts have a hypothesis, ancient Romans mined puzzolan from mining tunnels below. While sounding out of the subsurface, they find an historic mining tunnel. They have uncovered a vast system of caverns. The caverns could collapse, and along with them the buildings above. The only thing that can shed light on this dark world is to delve into it. They have to survey this maze. The researchers agree: They could fill the caverns in with cement, that would shore things up. These explorers will be the last people to lay eyes upon these historic tunnels. Soon they will be sacrificed to save the city's future.