The tunnels date back to the city's earliest days. Image Credit: CC BY-SA 3.0 Thalan
The tunnels were formed over many hundreds of years thanks mainly to quarrying, weathering and the tendency for the Romans to build new structures over the ruins of old ones. Some of the tunnels have been re-purposed multiple times as catacombs, a sewerage system and even as bomb shelters during World War II.
More recently however the tunnels have started to represent something of a nuisance as modern structures built over the top of them are becoming increasingly vulnerable to subsidence.
Over the last few years there have been several dozen incidents in which buildings have collapsed in to the disused voids beneath the streets.
To combat the problem, a group of geoscientists has undertaken an ambitious new tunnel mapping project to make it possible to more easily identify the points most at risk of collapse. Not only will this help to alleviate the risk to buildings above but it is also providing a unique opportunity to learn more about the vast labyrinth under the city and to find out just how extensive it actually is.