Digging under an existing city is hard. You have to thread your way through deep-driven building foundations, dodge or relocate underground pipes and cables, an

Digging under an existing city is hard. You have to thread your way through deep-driven building foundations, dodge or relocate underground pipes and cables, and maintain enough integrity to keep everything above from sinking while you do it. And (please don't take this the wrong way) a lot of places outside the US have been easier to build in because, over the past century, those places had to be completely rebuilt anyway following the massive devastation of two world wars. The US escaped that devastation on its own soil, but is paying an ironic price in that now it's difficult or even impossible to retrofit more modern infrastructure into our cities, because we still have all the old infrastructure that never got destroyed.