A buddy of mine manages a drain pipe mfg plant here in NJ. A couple of guys were parking on the property and riding their BMX bikes in the pipes. One day, my buddy saw their car, and had one of his guys pick up a massive pipe with one of thier loaders and block in the car. Pretty funny. There was a brief discussion about unauthorized entry, liability, possibility of "accidentally" flattened cars, etc. Just be safe, especially sudden downpours, and potential for asphyxiation due to unauthorized discharges to the system. Oh, and come up with a screen play on your exploits. Maybe something like Goonies. Or a South Park episode.