This is being discussed in Prague, but it is going to be very expensive. The city is very much not-flat, with significant altitude differences, a lot of alread

This is being discussed in Prague, but it is going to be very expensive. The city is very much not-flat, with significant altitude differences, a lot of already existing infrastructure under the surface (three extant lines of metro with fourth one being built, some road tunnels, parking spaces etc.), and a major river must be crossed. Plus, the rocks underneath are fractured and finicky. It isn't a nice big slab of granite, but a mixture of sediments, water and various primordial rocks. Quite hellish to put tunnels into.