The foyer of SCP-2329, October 3rd, 1914. Records show four Foundation personnel were present in the room when this picture was taken, one sitting in each chair.

Item #: SCP-2329
Object Class: Safe
Special Containment Procedures: SCP-2329 is to be contained in Provisional Site 27-2329; when not in testing all entrances to SCP-2329, including windows, must be securely blocked with solid metal plating. Several city blocks around SCP-2329 have been purchased by the Foundation and a security system (consisting of multiple points of surveillance and neutralization of intruders) should be maintained in this perimeter.
Description: SCP-2329 is a six-story, 60-room apartment building that was secured by the Foundation in 1908; see Addendum 2329-b for more information on the circumstances surrounding initial containment. It has been kept up to code with all the necessary functionality required by local government since that point but has not been altered in any other way.
On both a structural and a visual level SCP-2329 closely resembles the buildings surrounding it and individuals are able to enter and exit it on their own free will. Once inside SCP-2329, however, they lose all ability to consciously perceive other sentient life, effectively rendering every other individual invisible to them. Similarly, humans outside SCP-2329 will lose all ability to consciously perceive those inside SCP-2329, both by direct observation (e.g. watching through a window) and indirect observation (e.g. monitoring a security camera). Some perception still exists at a subconscious level as individuals inside SCP-2329 will make an effort to avoid bumping into one another.
This area of effect also extends to inanimate objects possessed by another individual (e.g. clothes, hand-carried objects); if an individual drops an object they were carrying it will become perceivable by all other individuals, although it will not be perceived to have suddenly appeared but rather that the observer simply had not paid attention to it before. It additionally extends to all sensory input, not just sight – test subjects are unable to hear sounds caused by other subjects, cannot smell one another, and cannot feel one another even if forced into such close proximity that physical contact would be unavoidable. Apart from this effect, individuals inside SCP-2329 appear to retain complete free will and consciousness, to the point of being puzzled by the lack of other people. This is validated by letters recovered from tenants of SCP-2329 prior to Foundation intervention, many of which complain of the antisocial nature of their neighbors.
The inability to observe all subjects within SCP-2329 has so far been an absolute rule; observers are unable to see any subjects in videos or photos, thermal imaging shows absolutely no increase in temperature with or without subjects, and motion sensors apparently never trigger. It is currently unknown whether this is due to the anomalous properties of SCP-2329 causing all instrumentation to fail or if the instrumentation is working as expected but observers ignore all output that would indicate the existence of sentient life.
Addendum 2329-a: Partial Experiment Log

Experiment 2329-7
Date: ██-██-████
Procedure: Pedometers were attached to two subjects, who were then asked to position themselves precisely at either end of a 10m line drawn on the floor of SCP-2329 and walk towards the other end. Neither subject was informed of the other's presence.
Results: Both subjects made it to the end, both recorded a distance traveled of 10.2m, and both asserted strongly that they had remained on the line the entire time.
Experiment 2329-16
Date: ██-██-████
Procedure: Subject was provided a television set and was told to tune into the local news channel within SCP-2329; no information about SCP-2329's anomalous properties was provided.
Results: Subject reported seeing only a series of empty sets, hearing only the broadcast's instrumental soundtrack, and being greatly confused by this.
Experiment 2329-22
Date: ██-██-████
Procedure: Shortly after a safety retrofit, 30 subjects were instructed to enter SCP-2329's elevator and then attempt to go to another floor.
Results: The newly-installed weight sensor triggered and prevented the elevator from moving. This was deemed sufficient evidence that the subjects continue existing in the same space-time as each other and simply ignore one another entirely, disproving an earlier contention by Dr. Warm that subjects were entering unique parallel dimensions when crossing into SCP-2329 and were thus literally not present in our universe.

Addendum 2329-b: Postmortem report on Arthur Morgan Ziggman

The corpse of Arthur Morgan Ziggman, age 53, was discovered in the hallway outside his room in SCP-2329 on August 21st, 1908. An interview with the landlord of the building, who lives offsite and was entirely ignorant of the anomalous nature of SCP-2329, revealed that Ziggman had filed numerous noise complaints about virtually every other tenant. The last such complaint had been filed approximately two years ago and he had heard nothing from Ziggman since. Based on written correspondence and interviews with tenants, the date of Ziggman's final complaint appears to be roughly six months before the start of SCP-2329's anomalous behavior.
The Foundation became alerted after tenants became frustrated at the police's “inexplicable” refusal to investigate; interviews indicated that the tenants became aware of the body at nearly the precise estimated time of death. Postmortem investigation indicated that the first injury Ziggman suffered was a broken hip which could be correlated to an indentation in a nearby wall. It was concluded that Ziggman was knocked over by one of the other tenants; the reason why he was unable to instinctively avoid this tenant is unknown. His other injuries indicate that he was struck about all points of his body, sustaining 22 broken bones along with dozens of contusions. The fact that these injuries occurred within rapid succession of each other around 7:00 PM, approximately when the other tenants would be returning home from work, indicates he was likely stepped on repeatedly and with force by the heavy work boots worn by his neighbors.

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