“Go to the big return that feeds into the left air filter and open the service door. We’re using the return system because it’ll have cooler air. Look close and you’ll see I’ve marked it with my handle.”

Jack stepped to the door and found the lever marked with Dud’s little black spot within a circle. He pulled it open and looked inside. Dark. Very dark.

“Dark, isn’t it. But not for long. To the right of the door is a light switch. Flip it.”

Jack did, and an incandescent bulb lit the inside of the duct—a square galvanized metal shaft, eight foot on a side. A dozen feet to his left it made a right-angle downward turn.

“Don’t stand there gawking, Jack. Get inside, close the door behind you, and start moving.”

Jack did and inched to the edge of the down shaft. Just below the lip, a metal ladder trailed down the inner surface of the shaft; its rungs were swallowed by the darkness beyond the cone of light cast by the single bulb.

“Use the ladder to get to the twenty-first floor. Don’t worry about the dark. We’ll take care of that as we go.”

“If you say so,” Jack muttered.

He swung over the edge and started down. As he neared the darkness below…

“The engineers who renovated this system were unusually considerate. Not only are there no motion detectors or grates in the ducts—something I’d recommend if I was trying to keep out people like us—but they placed a light on every floor, same as in the elevator shaft. But these have to be turned on. Keep an eye out to the right of the ladder as you pass each major seam. You’ll see a pair of light switches: One operates the bulb above you, and the other the bulb below.”

“Love those considerate engineers,” Jack said as he found the switches and hit the one that illuminated the section below.

“Conserve energy, Jack. Turn off the light in each section as you leave it.”

“You do it your way, Dud. I’ll do it mine. I like to see where I’ve been.”

“Turn me off until you see my handle on the twenty-first floor.”

Jack found the off switch and continued his descent without a running narrative. The only sounds were his soft, echoing footsteps and his breathing. Farther down he found a big “21” in red marker facing him through the rungs of the ladder. Dud’s handle hovered under the curve of the “2” like a floating eye.

Jack turned on the Walkman.

“Okay, Jack. If you’re at the twenty-first floor, it’s time to leave the big vertical and enter the laterals via that opening on your left. These get smaller as we go, and unfortunately they’re not lit for us, so you’ll have to turn on the headlamp.”