What got me thinking is out west, EVERYWHERE out west, there are massive drainage tunnel networks under the cities.
As teens we used to be able to get from one side of Tucson to the other, with very little time in daylight. The tunnels go under schools, malls, business complexes, and apartment complexes. Needless to say they go through multiple neighborhoods.
If imported or "domestically radicalized" scumbags are looking at tunnels for deviosity, it is disturbing.