Somewhat on topic, but in Colorado we have 6 of the original Titan I nuclear complexes. Abandoned since 1965, each complex has probably a mile of tunnels, huge underground rooms and 3 massive silos. They're absolutely fascinating to explore.

Some random photos from the last time we went in to one.

https://www.instagram.com/p/BLysXzHjG58/?taken-by=hamonacob

https://www.instagram.com/p/BK3fsXFjwE6/?taken-by=hamonacob

https://www.instagram.com/p/BCqaiG-KHGF/?taken-by=hamonacob

Happy to answer any questions for the curious.