Action Squad: Minneapolis Urban Adventurers
This was featured on metafilter, but they didn't mention this is a Minneapolis street gang! I may have met some of these guys. Reminds me of exploring steam tunnels at college -- but in those days Caltech allowed undergrads to have keys to many buildings, so it wasn't even trespassing (though it wasn't approved either).
My personal experience with urban adventures was limited to rooftop camping in the ancient days when I had far more time than money.