There weren’t any openings along the floor. The fittings around the steam pipes were tight. I didn’t spot any doors to slip under, no loose tiles in the ceiling. No way for anything to bubble up from the depths.

But in the farthest corner of the gym, the paneled walls themselves radiated cold.

I gave the wall a thump.

It was hollow.

Hearing the empty sound, I realized something about the darkened health club—it didn’t have any stairs down. The second level promised in the blueprints didn’t exist. Or it was hidden.

My duffel bag clanked against the concrete floor. From a pocket, I pulled the plans that Chip had printed for me, checking my compass. According to the blueprints, the sub-basement stairs were only a few yards away, on the other side of the wall.

The wood paneling didn’t give at all when I pushed; there was something solid behind it. Of course, my duffel bag was full of drills, hacksaws, bolt cutters, and a crowbar, or I could have just put my fist through the wall. But I still had to come back in my Sanitation costume to reclaim any rats my traps caught, and the staff doesn’t like it when you break their building.

I moved along the wall, pressing and thumping. The echoes were muffled, which meant that lots of crisscrossed S beams supported the paneling. The stairs were solidly sealed off as well, with no easy way in. Had they just abandoned a whole subbasement down there?

The wood paneling ended at a row of lockers—too heavy to move, even with my peep muscles. I tapped the floor with my feet, wondering what was hidden beneath. From the ceiling, the red eyes of security cameras glowed mockingly in the darkness.

Then I realized something: All of the cameras were pointed more or less at me. Were they tracking me?

I moved a few yards, back into the cold corner, but the cameras didn’t follow. They all stayed pointed at the same target—the row of lockers. Whoever had set the security system up didn’t care what happened in the rest of the health club, as long as they could watch that one spot.

I walked along the lockers, running my fingers against them, smelling the dirty socks and chlorinated swimsuits inside. The metal grew colder as I went.

In the center, one locker was icy to my touch, and through its ventilation slats that half-familiar scent—the one I couldn’t quite identify—floated on a draft of chilled air. I looked up at the cameras; they were all pointed directly at me now.

The padlock was an off-the-shelf Master Lock, though with four tumblers instead of the usual three, more expensive than the others. I knelt, cradling it like a cell phone to my head. As the numbers spun left, then right, I heard the tiny steel teeth connecting, the tumblers aligning … until it sprang open, as loud as a gunshot in my ear.

Sliding the lock off the hasp, I opened the door.

There was nothing inside—nothingness, in fact. No hanging clothes, no hooks or shelves, just a black void that consumed the dim light of the gym. A chill wind came from the darkness, bearing that same half-familiar smell, sharpened now.

I reached into the locker. My hand went back into the darkness and cold, disappearing into nothingness.

Let me get this straight about my night vision: When I’m at home, the only light I keep on is the red LED of my cell-phone charger; I can read fine print by starlight; I have to tape over the glowing clock face of my DVD player, because otherwise it’s too bright to sleep in my bedroom.

But I couldn’t see jack inside this locker.

There is something called cave darkness, which is ten times darker than sitting in a closet with towels stuffed under the door, covering your eyes with your hands—basically darker than anything you’ve ever experienced except down in a cave. Your hands disappear in front of your face, you can’t tell whether your eyes are open or not, random red lights seem to flicker in your peripheral vision as your brain freaks out from the total absence of light.

“Great,” I said.

Hoisting my duffel bag, I slipped through into the void.

The standard Night Watch flashlight has three settings. One is a low-light mode designed not to burn out peep night vision. The second setting is a normal flashlight, useful for normal people. The third is a ten-thousand-lumen eyeball-blaster intended to blow away peeps, scare away rat hordes, and generally indicate panic. Held a few inches from your skin, it will actually give you a suntan.

Switching on the tiny light, I found myself in a narrow hallway, squeezed between the foundation’s cement wall and the back side of the maniacally reinforced wood paneling. The floor was covered with little globs of something gooey. I knelt and sniffed and realized what I’d been smelling all along—peanut butter, mixed with the chalky funk of rat poison. Someone had laid out about a hundred jars of weaponized extra-crunchy back here. The bottom of the false wall was smeared with it to prevent the wood paneling from being gnawed through.

I stepped carefully among the gooey smears, and the hallway led me back to the corner where the missing stairs should have been. An industrial-strength metal door stood there, reinforced with yards of chain and generous wads of steel wool stuffed into the crack beneath it.

Steel wool is one thing rats can’t chew through. Someone was working conscientiously on the rat issue. Hopefully that meant Chip was crazy on the giant monster issue, and all there was down here was some peep’s long-lost brood.

The chains wound back and forth between the door’s push-bar handle and a steel ring cemented into the wall, secured with big, fat padlocks that took keys instead of combinations. To save time, I pulled bolt cutters from my bag and snipped the chains. As taut as rubber bands, they snapped loose and clattered to the floor.

Funny, I thought, chains don’t keep out rats.

Ignoring that uncomfortable fact, I gave the door a good hard push; it scraped inward a few inches. Through the gap, the promised stairs led downward into smellier smells and colder air and darker darkness. Sounds filtered up: little feet scurrying, the snufflings of tiny noses, the nibblings of razor-sharp teeth. An all-night rat fiesta—but what were they eating down there?

Not chocolate, was my guess.

I pulled on thick rubber gloves.

The gap was just big enough to squeeze through. As I descended, I kept one thumb on the flashlight’s eyeball-blaster switch, ready to blaze away if there was a peep down here. I couldn’t hear anything bigger than a rat, but, as I’ve said, parasite-positives can hold their breath for a long time.

The rats must have heard me cutting the chains, but they didn’t sound nervous. Did they get a lot of visitors?

At the bottom of the stairs, my night vision began to adjust to the profound darkness, and the basement eased into focus. At first I thought the floor was slanted, then I saw that a long swimming pool dominated the room, sloping away from me. The paired arcs of chrome ladders glowed on either side, and a diving board thrust out from the edge of the deep end.

The pool contained something much worse than water, though.

Along its bottom skittered a mass of rats, a boiling surface of pale fur, slithering tails, and tiny rippling muscles. They scrambled along the pool’s edges, gathered in feeding frenzies around piles of something I couldn’t see. All of them had the wormy look of deep-underground rats, slowly losing their gray camouflage—and ultimately even their eyesight—as they spent generation after generation out of the sun.

A fair number of ratty skeletons were lined up on one side of the basement, bare ribs as thin as toothpicks—as if someone had put out glue traps in a neat row.

There were a lot of smells (as you might imagine) but one stood out among the others, raising my hackles. It was the scent mark of a predator. In Hunting 101, we had been taught to call it by its active molecule: 4-mercapto-4-methylpentan-2-1. But most folks just call it “cat pee.”