I thought all kids did this kind of thing? When me and my friends were young this is one of the things we would do.
Our highschool had fallout tunnels that were unlocked from the outside and I dont think they knew it, but we did.
The elevator in town was unlocked in the back. We went up to the top on the manlift and in the tunnels underneath. That manlift is super dangerous. Its just a belt with a wood platform to stand on and a handle to hold, no cage or anything if you fall off you are falling over a hundred feet to your death. Now I can go up there with a key, without being a criminal. Two opposite pictures I took. On on the ground looking up the other from the top room.
There is a creek that runs under the town in a tunnel over a half mile long. Its called the Pocamo we used to ride bikes in there and race boats we made out if tin foil.