[avuserow]

> I'd be interested to know if any US cities have deeper tunnels

The downtown portions of Chicago's Blue Line[1] and Red Line[2] were primarily dug using tunnel boring machines in the 1940s. Here's a better view of the tunnels being not square[3].

Edit: infographic of various tunnels under Chicago, many of which are even deeper than these bored ones[4]

1: http://www.chicago-l.org/operations/lines/dearborn_subway.ht...

2: http://www.chicago-l.org/operations/lines/state_subway.html

3: http://www.railworks.com/track-project/cta-red-line-tunnel

4: https://uploads.disquscdn.com/images/32697b82560fbd40ec95ec1...