“I think it's just ingrained in human nature, you know?” says Hartman of the drive to venture below. “You wonder what’s down there.”

Hartman lives in in Tacoma, Washington, now, but grew up in Southern California. It was roaming around on his bike as a kid with friends, Goonies-style, that he discovered the dark urban waterways in his hometown.

“You take a 12-year-old kid, and show them an entrance of a tunnel? Like, they're going to go in," says Hartman.

Arriving in San Francisco later in life, he began exploring the city’s streets at night with his camera. One of those nights, a chance encounter with a manhole left open led him beneath the San Francisco for the first time — and sparked an adult passion for urban exploration.

Across the sleeping city, Hartman found entrances to dark, dripping tunnels, sloshing wet, that stretched for miles into the blackness.

“So much of it is just overgrown,” he says of those doorways. “You don't realize that there is a whole underground part of this thing.”

Like many urban explorers, Hartman says, he enjoyed the thrill of the hunt almost as much as the actual discovery.

“Like solving a puzzle,” he says. “It’s as much about solving the mystery and finding the thing.”

He used a mixture of publicly available records and maps, Google Earth and whispers from fellow urban explorers, who are notoriously secretive about their finds.

At least some of that is due to the risks of their enterprise. Bodily dangers aside, urban exploration represents “at best a gray area of legality in some places, and outright trespassing in other places,” as Hartman puts it.

This is the part where I tell you that this underground network Hartman risked bodily harm to venture into is no mysterious labyrinth built by shadowy figures.

It’s San Francisco’s huge sewer network.

Bay Curious is a podcast that answers your questions about the Bay Area. Subscribe on Apple Podcasts, NPR One or your favorite podcast platform.

A Complex World You Don't See

“I crawl through a lot of sewer pipes. That's basically my job,” says Megan Abadie, an assistant engineer for the San Francisco Public Utilities Commission wastewater enterprise. Her job sees her enter those same tunnels — legally — to make sure that this giant, intricate system filled with your waste keeps working the way it’s meant to.

There are a lot of misconceptions about the sewers, says Abadie. For one, what we surface-dwellers call "tunnels" aren’t truly tunnels — a term that specifically means a long run of pipe, bored out of the earth with only a few manholes attached. When we talk of the “tunnels under San Francisco,” we’re usually talking in fact about sewer mains.

San Francisco is 49 square miles, but contains over 1,000 miles of sewer mains, running under every block. What makes our system unique in California is the fact that it’s a combined system. Instead of stormwater and sewage water being separated in different pipes, as they are elsewhere in the state, in San Francisco it all flows into the same set of pipes.

This is a legacy of the city’s relative age, with the foundations of our modern-day sewers being laid during the Gold Rush — in what Abadie describes as “a very ad hoc system … people would build pipes to just connect to the nearest creek.” There are still some pipes under your feet that date from the 1840s, she says.

Just like in New York — another old, dense city — it was too hard to rip up San Francisco’s sewer network to replace the old system with secondary pipes. So we’ve repaired and adapted our old system, which is why this city still has those big, wide sewer mains ... that people can’t seem to stay out of.

A Lethal Labyrinth

“There's a lot of things that can happen in the sewer that can actually kill you pretty easily,” Abadie reminds me.

For one thing, there’s the risk of drowning down there. Because of San Francisco’s steep topography, Abadie and her colleagues never enter the sewers if there’s so much as a drizzle of rain anywhere in the city.

“If you're in a large pipe at the bottom of a hill, it doesn't take much for a big slug of water to hit you, even if it's not raining very much where you are,” she says.

Then, there’s the danger of toxic gas: namely hydrogen sulfide, produced when organic material (waste matter, seaweed) starts to decompose. At low levels, it has a distinctive smell of rotten eggs. At higher levels, it affects a person’s sense of smell entirely, and can knock you out — and kill you — within minutes.

On top of that, there’s the threat of simply getting lost, injured or both in the sewers. Abadie and her fellow inspectors are equipped with accurate maps and supported by a large chain of people both below and above ground — weather spotters, medics.

“When I go into the sewer system I know exactly where I am. ... You go into a pipe that you see sticking out somewhere? Open up a manhole? You're not going to know where you are,” Abadie says.

After hearing this, I had zero intention of attempting to explore the sewers alone for this story. But I couldn’t resist asking Megan to take me down to see an underground place that Sierra Hartman had told me about.

A Trip into the Underworld

It looked more like a cave than a sewer, Hartman says. And I knew urban explorers like him would spend months, even years, trying to track down its precise location — because of how striking it looked, and how it leads right out to the Pacific Ocean.

Abadie knew exactly the place Hartman meant, and asked me to wait until the timing was just right, when it’d be safe enough at low tide, with no chance of rain. That timing turned out to be very early in the morning on the Fourth of July: the lowest tide of the year.

As Abadie’s crew secured a harness and waist-high waders to my body, she explained why we’d be taking gas meters and oxygen masks down there. Even though the fast flow of the system we’d be entering would lower the hydrogen sulfide risk, “you can go into a sewer that's been fine every single time, and one year something can be different,” she says.

With safety equipment secured, we were lowered one by one into the tunnel by rope, down a tall, rusting ladder until we finally, reached the bottom of the sewer with a splash. The water reached our knees. Ahead, through the humid, misty air, was a long, high tunnel that seemed to stretch for miles in front of us. Down there in the darkness was that “sewer cave” — and the ocean. During the rainy season, Abadie reminded me, the tunnel we stood in would have been full of water.

Surprisingly, the sewers don’t smell how you might fear they would: the odor is agricultural, like a farmyard smell. Yet no matter how pleasant this surprise, wading through high sewer water in such humidity quickly becomes exhausting, like walking through deep snow.

As we walked through the tunnel, our voices echoing off the walls, Abadie told me about her first entries into the sewers after she started working for the city in 2011. The underground network, she says, reminded her of the vast Mines of Moria in "The Lord of the Rings."

“I thought it was really cool. I even thought it was cool seeing a little turd float by! I mean, that's not something everyone gets to see,” she says.

As we got closer to what I’d come to see — that cave — the crashing of the Pacific Ocean suddenly grew louder. Looming in front of us, there it was: What looked like the tall, wide mouth of a cave, deep under San Francisco, carved from dark, jutting rock and yawning into more blackness.

“This,” says Abadie with some pride, “is definitely the most scenic and beautiful combined sewer overflow in San Francisco.”

Passing through the cave, we had to stoop to get through the last part of our journey, our helmets scraping the ceiling. We were now inside the discharge pipe: the way the system can safely get water out during heavy storms, while providing primary-level treatment, when the usual storage areas under the city are full to the brim.

At the end of the pipe, the waves we could hear crashing close suddenly became visible, as I found myself looking out at the ocean, framed by rock. After hours underground, it was now daylight out there. That entrance onto the water is, unthinkably, how some explorers try to get in here, via a tiny strip of beach that opens up only for a brief period of time.

As the tide started to rise, the waves started to crash further and further into the pipe toward us, and we knew it was time to go. As we moved back through the tunnel, the difference in smell was palpable: The people of San Francisco were waking up, and were starting to use their bathrooms.

After being attached to the rope and hauled out of the darkness and up through the manhole again, I was suddenly out of the city’s underworld. Exhausted, after hours trudging through sewer water, the call of the underground was only more apparent to me.

So what could people do, I asked Abadie, if after hearing the truth about the darkness and danger down there, they still couldn’t resist the lure of subterranean exploration?

“We have a lot of people retiring here. You can come work for us!” she says. “We will get you into sewers. It'll be awesome. Your passion can actually get you paid to explore sewers.”

“Or become a public radio reporter,” she added. “Those are two ways that you can get into sewers and not die.”