The body was in alley three blocks from my office, behind a dumpster that smelled like a drunk man's regret. I found him because the guy who owned the body—Vinny the Knuckles, which is not a name anyone chose for themselves but is exactly the kind of name that chooses you—had come to see me two days before he died and asked me to keep an eye on his kid.

Vinny looked like a man who had been punched by a man who had been punched by a piano. His face was a map of every bad decision he had ever made, drawn in scars and swollen cartilage. He sat in my office chair—which was already uncomfortable without someone sweating through it—and told me, with the desperate earnestness of a man who knows he is running out of time, that something was happening in the tunnels under Detroit.

"There's creatures down there, Mercer," he said. "Little green things, like monkeys but smarter. They're killing people. The police don't know, the Feds don't know, and if you tell anybody I told you, I'll deny it and you'll deny it and we'll both be fine."

I told him I didn't do supernatural work. He said these weren't supernatural. They were biological. Something new. Something that had always been there and nobody had seen.

Then he left, and two days later he was dead, and the medical examiner found a fragment of black mineral embedded in his sternum that neither the coroner nor I could identify.

I took a piece of the mineral in a paper bag and held it to the fluorescent light of my office. It absorbed the light rather than reflected it, drinking it in like a sponge. I put it in my desk drawer and tried to forget about it, which is what I do when things happen that I can't explain.

I forgot about it for exactly three hours.

The next lead came from a cop named Rinaldi, a beat guy who owed me a favour from the '22 strike. He met me at a bar on Woodward Avenue and told me, over a beer I didn't want, about the homeless guy who had been found in the old subway tunnels under downtown Detroit. The guy was alive but not making sense—babbling about "the green people" and "the signal from below" and "they're not monsters, they're just hungry."

I went down to the tunnels.

Detroit's underground is a city within a city, a labyrinth of abandoned subway tunnels, Prohibition-era speakeasy cellars, steam pipes from buildings that have been demolished, and storm drains that lead nowhere. I had been down there before, during the dry years when the city couldn't afford to maintain what it had, but I had never seen anything like what I found on that night.

The tunnels changed as I went deeper. The concrete walls gave way to natural limestone. The artificial light from the street-level grates gave way to something else—a faint, greenish luminescence that came from the walls themselves, from veins of some mineral embedded in the rock that pulsed slowly, rhythmically, like a sleeping thing breathing.

And then I heard them.

Not with my ears. With my teeth. A vibration so low it registered in my bones rather than my eardrums, a hum that set my molars tingling and made the hair on my arms stand up. It was the same frequency I had felt when I held the black mineral to the light.

I followed the hum deeper into the tunnels, past layers of abandonment that went back a century, and I found them.

They were not green monkeys. They were not monsters. They were a form of life so alien that my brain kept trying and failing to translate them into human categories. They were perhaps two feet tall, with limbs that had too many joints, and skin that was the colour of moss after rain. They moved with a fluid grace that made my human walking look like a child's stumbling.

They stopped what they were doing when I appeared. Seven of them, clustered around a pool of black water, and they looked at me with eyes that were entirely black and entirely intelligent.

One of them stepped forward and made a sound—a low, complex frequency that I felt more than heard—and something in my chest responded. A warmth spread through my ribs, my breath quickened, and I felt my body doing things it had not done before: adjusting, adapting, becoming more sensitive to the darkness around me.

They were not hostile. They were curious. And they were making me like them.

I backed away slowly, keeping my eyes on them, and the one that had stepped forward lowered its head in a gesture that might have been respect or might have been a warning.

I climbed back to the surface and went to my office and poured two fingers of bourbon and did not drink them. I sat in the dark and felt the changes moving through my body like a slow tide. My vision was already adjusting to the dim light. My hearing was sharper. My instincts were... different.

Not better. Not worse. Different.

The next morning, Rinaldi called and told me three more people had gone missing from the downtown area. I told him I was sorry. I hung up the phone. I poured the bourbon down the sink. I got my coat and my gun and my notebook and I went back to the tunnels, because that's what I do. I don't know why I do it. I just do it.

The tunnels called to me, and I answered.