This stuff is amazing. I never really knew this was a "thing".
Anyway, me and my friends did some of this stuff as teens. Went to "peabody's tomb" in glen ellyn, illinois & the "ovaltine factory" in villapark, illinois.
Both come up on google with little info, but they were pretty amazing places. Looking back I'm just surprised none of us fell through a rusted stairway or anything.
http://www.infiltration.org/ - Great site! I'm thinking of picking up a subscription!