[_notreallyme_]

> 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.