Thanks for all the comments guys..
All the underground photos are from the Catacombs in Paris..
Essentially, When Paris was being constructed, they mined limestone out in huge mines 25m below the surface of Paris - huge vast chambers. Once construction was complete, people forgot about the void beneath their feet.
As it became more developed, Paris had its memory jogged when entire streets suddenly started falling into the mines. This continued throughout the 13th, 14th, 15th and 16th districts, until the reining king of the time decided to form a group (ICG or IGC -I can't remember) to fix the problem. They formed in 1777, and late that year, began work in the quarries, adding walls and tunnels and reinforcements, essentially honey-combing the void, to reinforce it. Work continued as late as 1845, to where it is today, where places still need to be reinforced with more concrete to stop cave ins - yet there are sections where the walls bow like arcs under the weight above.
During the period of construction, Paris had issues with disposing of corpses - and so resorted to dumping the bodies of the dead in certain sections of the catacombs - the ossories - which are the pictures you saw above, as well.
Unlike what people think, the catacombs are incorrectly called so, and a 'technically' known as the 'quarries' - and do not have bones all throughout them.. maybe 1% of the 300+km of tunnels..
lesson over!