The concept of urban exploration was first given a name in San Francisco in the late ’70s by a group called the Suicide Club.

The club was known for all manner of shenanigans including filling cable cars with naked riders, climbing the Golden Gate Bridge, and throwing huge parties in the sewers.

The boom and bust mentality of this city has been an ongoing trend since the gold rush days.

So much of the city has been built in consecutive layers that traversing large parts of it entirely underground is not as difficult as you might think. Some of these places are relatively well trafficked by graffiti artists.

As for the secret society known as The Suicide Club, there’s some additional information on it available from Wikipedia.

Though best known for its bridge climbing exploits, it had its fingers into a number of bold activities from infiltrating the American Nazi Party, the aforementioned thirty members riding naked on San Francisco’s cable cars and some really elaborate games in strange locations.

The name was an inspiration from three stories written by Robert Louis Stevenson where men who want to die belong to a club whereupon each evening one is randomly selected for death.

Via The Bold Italic for tons more photos.