I lived in Colorado Springs from 1984 to 1990.
That place was wide-open and the owners were OK with people exploring and even writing graffiti. There were several, very long concrete tunnels. It was just really cool place to run around in.
At some point around 1988 or so, something "bad" (I forget the details) happened there and up went a fence and lots of no trespassing
signs. There was a write up about the incident in Gazette-Telegraph. If was more motivated, I'd go look it up.
When the Golden Cycle Mill was open and working, it was the largest cyanide mill in the world. Hence lots of ground pollution and why it has been sitting vacant for a very long time.