But younger Romans are less aware of the geological hazard under the city, Kysar Mattietti said. And few realize the quarries' extent.
"Since they weren't serving any use, people tend to forget what can be a problem," Kysar Mattietti said.
Now, Kysar Mattietti and other geoscientists are using laser 3-D scanning to search for hidden weaknesses in the tunnels. The researchers also enter the tunnels through manholes and map the labyrinth by hand once they're sure the area is safe.
"There might be cracks, so they will be showing as veins almost, or openings, so we map the openings and map any kind of detachment," she said. In some spots, the ceiling of the tunnel sloughs off like cracking plaster. In others, there are total collapses — sometimes not reaching quite to street level, but leaving very little ground between the surface and the void.
"It's interesting, because at times when you are down there, you can hear people on top," Kysar Mattietti said.
To fix critical points, city officials seal off the unstable point and pour mortar into the tunnel, filling the entire void instead of simply patching over the top.
"What the municipality wants to do is to basically have a map of the risk so at that point they can on their side decide what kind of intervention needs to be done," Kysar Mattietti said.
The geoscientist presented her mapping work this October at the annual meeting of the Geological Society of America in Denver.
Most of the quarrying is under the southeastern area of the city. Kysar Mattietti and her team are currently mapping three sites considered at particularly high risk of collapse. The need will likely only increase as natural erosion works its destructive magic in the quarries.
"A crack never stops on its own," Kysar Mattietti said. "It always gets bigger."
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