> Paris is very complicated underground. Besides the quarries there are the sewers, catacombs, some underground lakes, reservoirs, etc. It is definitely a complicating factor when planning new metro tunnels.
I wonder if the French are considering digging underneath all those obstacles? Seems to be the way London has gone with the Elizabeth Line. Although that may still be shallower than Paris' quarries and lakes.
> I wonder if the French are considering digging underneath all those obstacles
Paris has been doing it for decades. All new lines in Paris (such as metro line 14, opened in 1998, RER E) have been dug by tunnel boring machines, and are at a depth below 20m. Same goes for all new lines outside of Paris proper, like most of the Grand Paris Express 200km+ new lines.
> I wonder if the French are considering digging underneath all those obstacles?
The problem is the Seine phreatic zone, which starts usually between 15 to 25m below the surface. Some GRS galleries are actually completely inundated and others have a level of water that varies between the seasons.
In order to have some metro going underneath the Seine river, they had to freeze it first. It is not an easy task, so there must be a real advantage to going under the Seine.
I wonder if the French are considering digging underneath all those obstacles? Seems to be the way London has gone with the Elizabeth Line. Although that may still be shallower than Paris' quarries and lakes.