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.