I've been doing maintenance in the Underworld for eleven years. I know these tunnels. I know which lights flicker, which pipes leak, which junctions flood in spring. I know the sounds — the hum of the air circulators, the drip patterns, the way your footsteps change when you cross from concrete to bedrock. I know all of it. I do not know what is behind the door in Tunnel K-9.

The door is steel, industrial, the same type used throughout the Underworld's mid-depth infrastructure. It has a standard mechanical lock. I have the key. The first time I opened it, in January 2198, it led to a utility closet containing pipe fittings and a mop. The second time, in March, it led to a staircase descending at least four flights. There is no staircase behind that wall. I checked the blueprints. I closed the door and opened it again. Utility closet. Pipe fittings. Mop.

I have opened the door forty-six times. I keep a log. It has been the utility closet thirty-one times. It has been the staircase eight times. It has been a long, dark corridor with no visible end three times. Once it opened onto a room full of filing cabinets, floor to ceiling, with labels in a language I don't read. Once it opened onto what I can only describe as outside — sky, horizon, grass — which is impossible at depth 6. The air that came through smelled like rain and distance. I stood in the doorway and looked at the sky for about two minutes before I closed the door. When I opened it again: utility closet.

My colleague, Saoirse Ndiaye-Hoffmann, opened the door in my presence in August. For her, it led to a small bedroom with a single bed, a nightstand, and a glass of water. She said it looked like the room she grew up in. I was standing behind her. I saw the utility closet. We were looking through the same door at the same time and seeing different things. I have not reported this through official channels because the last person who reported an inexplicable anomaly through official channels was reassigned to surface sewage inspection. I am logging it here. The door is still there. I still open it sometimes. I am not sure it opens for me. I think it opens for whoever it wants to show something to.