If you grew up on a steady diet of Teenage Mutant Ninja Turtles, then you're already deeply aware of the exciting and terrifying possibilities of the underground. The dark sludgey atmosphere of underground subway tunnels has captured the imaginations of generations of people, who see them as mysterious places to explore or do drugs in peace.
As anyone with experience knows, traveling into dark underground tunnels, bomb shelters and creepy attics is not for the faint of heart. You have to go ready to ruin your shoes and sense of smell, while potentially traumatizing yourself with what you find. Still, the stories are worth it in the long-run.
There's an abandoned and boarded up WWII fort in the southern part of Belgium, that we often sneak into with the scouts. Getting in there requires scaling a sheer wall (where we've placed anchoring points for ropes and climbing gear) next to a relatively busy road. So you're being super quiet, making no light and cowering every time a car passes by so he doesn't spot you in his lights. The atmosphere is set.