Chicago Tunnels are quite extensive. How many tunnel systems are actually from the old world that they've now incorporated into the system of today.

The Mud Flood (ARCHIVE) CHICAGO: Before and After
What happened to Chicago? It was a place of industry and mechanical engineering, leading the world in automotive innovation since 1862. Here are some pre-flood images and engravings of Chicago before the flood and images of the aftermath. It is a shame what happened to Chicago and it's city founders

In 1892, a mud flood destroyed cities across the globe, restructuring power and an opportunity to take over cities and societies and erase history. Egypt suffered the same fate as other cities across the globe. A great civilization wiped away, leaving only a trace of what once was, eroded pyramids and sphinxes were just a few of the victims. Monuments and obelisks, ransacked by governments around the world to be put away out of sight
https://www.youtube.com/watch?v=72N9NyO8_Oo