Rome was neither built in a day, nor was it always built over the best places. Humans have been living in the city of Rome for thousands of years and have a habit of building over former structures, such as the tunnel you’re looking at. When the city was expanding during imperial times, substantial portions built from volcanic rock mined just south of the city. This volcanic rock is what we’d call “pyroclastic material”. It was erupted out of volcanoes as ash and formed into rock on the surface. This type of rock has a lot of space in it (called porosity), making it lightweight and easier to carve than other stones that could be used for building, but also strong when used as building stone. The Romans quarried large amounts of this rock to build their cities, leaving a series of long tunnels on the southern side of the city.
Today, these tunnels sit beneath the city, beneath the streets and walkways. They’ve sat there for 2000 years in some cases, but the weight of modern civilization is becoming too much. In 2011 there were 44 collapses into tunnels like this; so far in 2013 there have been 83. Geoscientists are working on the problem. Using 3D-scanning lasers (now that just sounds awesome on its own), scientists led by a team from George Mason University in the U.S. are mapping out the tunnels and trying to locate areas that are prone to collapse, considering the rock types, thickness, and how erosion has weakened the rocks over time. Via Earthstory