There is an extensive system of tunnels under the campus of the University of Notre Dame (yes, THAT place). And do you have any idea what's under their football stadium?

A rifle range.

Built in the 1940s as part of the Navy's V-12 program it was used up until at least the late 1970s by the ROTC; it's been closed because of lead hazard since. I understand from someone who shot there that it damned COLD down there.