I wonder what those other cities are built on. The Second Ave subway's bedrock is schist [1] -- which is an extremely strong, durable type of rock.

A quick glance at this [2] engineering book indicates Copenhagen is limestone. Apparently the same is true for Paris. Any engineers here who want to chime in about how that affects cost?

[1] http://mobile.nytimes.com/2012/08/05/magazine/tunneling-belo...

[2] https://books.google.com/books?id=6UAqS62NJDMC&pg=PA2616&lpg...