Crossing the cities
The topography of some cities does not offer any other solutions for crossing them than going under the surface. Such is the case for coastal cities, surrounded by mountains, where virtually all the free space at the surface has been long ago occupied.
This is the case of the city of Marseille in France. A large number of tunnels enable the access by road to the city, the first one being built in 1967, another one being an old railway tunnel converted into a road tunnel and the most recent being put in operation in 2007. Some others are still under consideration.
On some occasions, old surface infrastructure is put underground so that new spaces can be created at the surface, creating a more environmentally friendly solution that frees space for different uses.
One example are the new tunnels created in Madrid for burying one of the main ring motorways of the city, the M30. With diameters of around 15 meters, these tunnels have been excavated in the soil by means of the biggest TBMs ever used, at the time of the construction. Nowadays, green parks and calm spaces lay where formerly thousands of vehicles went through the city.