At my university, there were old, disused steam tunnels connecting all the older buildings on campus.
A small group of us explored them, and we even found an unlocked path into the University President’s office. At this point, we realized the nature of our trespassing, and possible consequences, so didn’t press our luck.
We weren’t the only ones. We found graffiti and beer cans / liquor bottles from as early as the 1940s down there.
One visit to the steam tunnels was enough for me to realize I was in over my head.