By Sierra Hartman

The only time I’ve felt like this whole urban-exploration thing might be a bad idea came about a year ago. I was walking through the top floor of a huge abandoned structure, and the floor started making a crunching sound. Normally, this wouldn’t have been a big deal, but I had just climbed 10 flights of stairs, and I knew by looking through a few holes that there was nothing between me and the ground floor but 100 feet of air. The structure was built in 1942, and I had no idea how long it had been since anything larger than a rat had walked through it. My sense of mortality suddenly became very apparent. I stuck to the solid bits of floor and made it out all right, but just thinking about it still makes my palms sweaty.

That sort of thing doesn’t happen underground. Don’t get me wrong — there’s a whole other list of perils to encounter in the tunnels under San Francisco, but at least I know the floor isn’t going to fall out from under me. More likely is the chance that I’ll come upon some old access ladder that’s been rusting for 50 years, and the rungs are about two pounds away from snapping, or that I’ll find that ground that was previously solid has been washed out by the tide and is filled with random, chest-deep holes.

The worst, though, is hydrogen sulfide. It’s heavier than air, corrosive, highly flammable, and very deadly. It comes from decaying organic matter in low-oxygen environments, e.g., sewers. It smells like rotten eggs, but it also deadens scent receptors, so if you smell it for a while and it goes away, you can’t be sure if it’s actually gone or if you just aren’t sensitive to it anymore.

The sewers built around San Francisco in the early 20th century dumped straight into the ocean. Most of them have been rerouted to treatment plants. A few were left as emergency overflows.