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.

I found a dead rat once near a sloping intersection of two tunnels. It was whole and relatively untouched. I went back two weeks later, and it was nothing but bare bones and hair.

Having the right tools and being smart about it does a lot to mitigate these risks. I’ve been fortunate enough so far that the lessons I’ve learned have not been too hard. I’ve also met a few people along the way (above and below ground) who have imparted valuable wisdom. Some of the stories I’ve heard are hard to believe, but some of the things I’ve seen are too.

The early box sewers of the 1800s were wretched masses of filth. Surprisingly enough, though, the latent nutrients in the decomposing effluent fed a whole ecosystem of plants and microorganisms.

Everything gets weird underground. In a mile-long tunnel with no obstructions, voices echo so much that you can’t understand a word until you’re right in front of someone. The temperature can be 30° higher even in the middle of the night, and humidity hangs in layers dense enough that they can be moved­­­­ with your hands.

In the relative stillness of an untouched tunnel, decomposition occurs in an unnaturally delicate way. This spider, still suspended in its web, died and was covered in a thin layer of mold.

Darkness in a chamber as big as a warehouse is different from darkness in a bedroom. In the absence of sensory input, your mind fills in the gaps. You’d be amazed by how quickly the distant trickling of water can sound like voices that shouldn’t be there. At the end of a long night under the city, finally climbing back out into the clean, foggy air is like waking from a bizarre dream.

One tunnel in particular was like a museum of graffiti. A number of pieces were over a decade old, dating back to the ’80s and ’90s.