Per 
Item #: SCP-XXXX
Object Class:Euclid
Special Containment Procedures: SCP-XXX is to be securely installed in a reinforced door frame within Site-XX. The door frame is to be monitored by at least two security personnel at all times. SCP-XXX must be checked regularly for any signs of damage or tampering. Should SCP-XXX need to be moved, it must be relocated to a new door frame within ten minutes to prevent fragmentation. Any instances of SCP-XXX-3 encountered must be contained and handled according to their sentience status.
Description: SCP-XXX is a six-foot white door made out of wood, featuring a distinctive red door knob. When opened, SCP-XXX serves as a portal to a pocket dimension that is an endless hallway of doors. Each door within this hallway is unique and, when opened, leads to various times, places, or universes. Notably, the appearance of each door within the hallway correlates with the location it leads to; for example, a destroyed looking door might lead to an abandoned house.
Inside the pocket dimension, text is inscribed on the floor in the native language of the observer. The inscription reads: "Welcome to the door to all doors. If you are stuck or don't know where you are just knock on the ground three times to go to the door you came in from . If you are stuck in a different world, knock three times on the door you came in, and it will open a portal back here."
These instructions function precisely as described. Upon knocking on the ground three times, SCP-XXX-2, a humanoid figure composed of black gas, will appear and transport the individual back to the original door they entered through. SCP-XXX-2 only manifests when summoned in this manner.
Various hostile entities, designated as SCP-XXX-3, have been observed within SCP-XXX. The origin of these entities is currently unknown; they may originate from within SCP-XXX or have access to similar doors. SCP-XXX-3 entities have been known to breach into our world via SCP-XXX. In the event of encountering SCP-XXX-3, the entity is to be detained, and SCP-XXX should be relocated to a different door frame. Sentient SCP-XXX-3 entities are to be interrogated and housed in individual containment cells. Non-sentient entities are to be subjected to further research and experimentation.
SCP-XXX has the capability to be moved to different door frames, which alters its position within the pocket dimension. If SCP-XXX is not placed within a new door frame within ten minutes of removal, it will break into three to five pieces. Subsequently, within five to ten hours, the nearest door will transform into SCP-XXX.
Addendum XXX-1: Due to the unpredictable nature of SCP-XXX-3 entities and the potential risks they pose, all personnel interacting with SCP-XXX are required to undergo specialized training and must adhere strictly to containment protocols. Further research is ongoing to understand the full capabilities and origins of SCP-XXX and the entities it houses