The Bunker Room is located on the far end of the Mirror room past the stairs where the wall is normally fully bricked over in vanilla gm_construct. A second entrance is accessed through the long section of the Tunnels that contains a transmission hole and connects the Submarine Room and Spawn with the Dark Room and Building C.
The Bunker Room is relatively empty except for a raised section on the left side that provides access to the Tunnels, with a staircase added for travel between the two levels. You can go underneath the stairs but so far there hasn't been anything hidden there. The wall running along the Tunnels has windows for players to peer in from both sides, as well as giving them a glimpse of Dark Kleiner if he appears there.
The only other notable feature is a hidden miniature dark room on the right side of the room which cannot be seen when noclipping to keep it hidden before players find it. To gain access to it, the player must attack the bottom right section of the wall, which will break and reveal the room's entrance and will remain open in subsequent sessions.
- A baby doll globe can be found on the raised section leading up to the Tunnels in the opposite corner near the stairs.
- Entering the mirror dimension reveals two blocks of text next to the Small Dark Room's entrance reading "ΓYES! Prohibited Materials Ahead. DO NOT O PE N" and "ANCIENT EVIL INSIDE DO NOT ENTER".
- The first block of text seems to imply some sort of testing was conducted with the room, or that it was being used to store hazardous or anomalous materials inside.
- The second block of text alludes to the room being used to contain an unknown anomaly or entity at one point.
- It is unknown if the player opening the room allowed the entity to escape or if it had already escaped on its own prior to the player's arrival.
- Addons that can deal map-wide damage such as HBOMBS can break the Small Dark Room's Wall without the player's knowledge. Since the entrance being open persists, it is possible for the player to not even realize the room was initially hidden or what caused it to appear.