SCP-087 is a stairwell that never reaches a bottom no matter how far one descends. Landings repeat every thirteen steps without end, and a flashlight beam cannot push the darkness far away. A sound like crying drifts up from somewhere below, and explorers encounter a faceless being designated SCP-087-1.
The heart of the horror is endlessness coupled with the limits of sight. The claustrophobia and unknowing of having to keep descending without knowing what waits just past the dark builds the dread. Because what lies beyond the reach of light can never be confirmed, it is a case where the limit of observation itself becomes the fear.