'Secret' Labyrinth of Roman Tunnels Mapped

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Deep under the streets and buildings of Rome is a maze of tunnels and quarries that dates back to the very beginning of this ancient city. Now, geologists are venturing beneath Rome to map these underground passageways, hoping to prevent modern structures from crumbling into the voids below.

In 2011, there were 44 incidents of streets or portions of structures collapsing into the quarries, a number that rose to 77 in 2012 and 83 to date in 2013. To predict and prevent such collapses, George Mason University geoscientists Giuseppina Kysar Mattietti and scientists from the Center for Speleoarchaeological Research (Sotterranei di Roma) are mapping high-risk areas of the quarry system.

The mapping is important, Kysar Mattietti told LiveScience, because through the years, Roman citizens have taken the patching of the quarry systems into their own hands. [Photos: The Secret Passageways of Hadrian's Villa]

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"The most common way is to take some big plastic bags and fill them with cement and stick them in the holes," she said.

Lucky geology

Volcanism created the land Rome was built upon. These volcanic rocks, or tuff, were a boon to Rome's earliest architects, who soon learned the tuff was strong and easy to carve into building blocks. Lighter, less compacted volcanic ash was used as a main ingredient in mortar.

The first Romans were savvy, Kysar Mattietti said. The geoscientists quarried outside the city, and found that even when the suburbs began to encroach over the quarries, the ancient Romans knew to keep the tunnels narrow enough so that the ground above was still supported.

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But two things worked against the long-term stability of the tunnels.

The first was Mother Nature. As soon as the rock is exposed to air, it starts to weather, Kysar Mattietti said. The second problem was human. Later generations kept building, using the same quarries for rock and widening the tunnels beyond their original size to create new structures above them.

Secret passageways

The tunnels are something of an open secret in Rome. Over the years, once quarrying ended, people repurposed the underground labyrinth as catacombs, for mushroom farming and as an unofficial sewer system. During World War II, people used the tunnels as bomb shelters.