For people like me, there are few finer pleasures in life than a night spent grazing on Doritos in front of the TV. But other kinds of people seek bigger thrills, which is why I am standing on New York’s Williamsburg Bridge at 3am with Miru and SeungJung Kim, fearless Korean-American sisters who have talked me into climbing one of the bridge’s steel support towers, just for laughs.

Miru and SeungJung are part of a loose-knit tribe of thrill-connoisseurs called “urban explorers” (also known as infiltrators, guerrilla urbanists, building hackers, and urban spelunkers, depending on who you ask). These are people for whom scaling bridges and roaming subway tunnels, storm drains, derelict factories, sewers, steam vents and other forbidden infrastructure of the city is considered fun. No place is too improbable or dangerous. In fact, if urban explorers have a unifying philosophy it is simply: “Danger is our business.”