**__SCP-9921 “The Double Door”__** **Containment Class**: Euclid **Disruption Class**: Vlam **Risk Class**: Danger Access Level 3 **Special Containment Procedures:** SCP-9921 is contained in a reinforced, climate-controlled containment chamber at Site-■■. The chamber is to remain sealed at all times, except during authorized testing or maintenance. SCP-9921 is to be physically restrained in closed position using a triple lock mechanical bracing system and two independent electromagnetic locks. **Description:** SCP-9921 is a freestanding, non-anomalous wooden door, measuring approximately 2.1 meters in height and 0.9 meters in width, mounted within a metal frame of unknown origin. The door exhibits no anomalous properties while closed. When opened, SCP-9921 functions as a bi-directional spatial threshold connecting to two distinct environments, depending on the direction from which it is opened: When opened from side A, SCP-9921 leads to a seemingly infinite desert environment. (Hereafter SCP-9921-A). When opened from side B, SCP-9921 leads to a seemingly infinite tundra environment. (Hereafter SCP-9921-B). Both environments exhibit normal gravity and an atmosphere breathable by human life, though temperatures in SCP-9921-A regularly exceed 45°C, while SCP-9921-B maintains average temperature below -30°C. Once an individual fully passes through SCP-9921 into either environment, SCP-9921 remains physically present and accessible. However, closing the door renders return to baseline reality impossible unless SCP-9921 is reopened from the Foundation side. SCP-9921 may be opened from within SCP-9921-A or SCP-9921-B; however, doing so shows the door is functioning as a normal wooden door opening onto the surrounding environment. The mechanism in which SCP-9921 prevents internal exit is still unknown. **Addendum 9921-1: Recovered Individual** On ██/██/20██, SCP-9921 was opened under controlled conditions following reports of knocking from the interior-facing side of the door. Upon opening, Foundation personnel encountered a human male standing immediately beyond the threshold, later identified as a civilian reported missing ██ years prior. The individual demonstrated advanced dehydration and heat-related injuries but remained conscious and cooperative. When questioned, the subject correctly described several properties of SCP-9921-A that had not yet been officially documented by the Foundation. The subject expired ██ hours later due to heatstroke. No further knocking events have been recorded.