The Quarry Threshold (also known as the Black Rock Quarry) is a Threshold located in the Maintenance Sector of the Oldest House. The Federal Bureau of Control uses the quarry to mine for Black Rock, a material highly valuable to them due to its ability to negate paranatural effects. As a Threshold, the Black Rock Quarry is connected to an alternate dimension, the night sky of which can be seen from the quarry.
The Black Rock Quarry consists largely of various black pillars, some of which are floating in the sky. The FBC has set up mining equipment across the quarry to excavate Black Rock. A valuable Black Rock Prism is located underground and can be excavated by Jesse Faden with explosives. The Black Rock Quarry can be reached through an elevator on the right side of the map, near to where the NSC energy converter is labelled, and slightly above where the Ventilator is labelled.
The Black Rock Quarry is a relatively stable Threshold compared to others in the Oldest House; as such, the FBC has established relatively long-term research and excavation equipment in the area. The Black Rock mined in the Quarry is used across the Oldest House for a variety of functions; Firebreaks are lined with Black Rock, and Altered Item/Object of Power containment crucibles are also often fortified with Black Rock. The Quarry is also the location of the Foundation, a mysterious location which is implied by the Board to be connected to the origins of the Oldest House and themselves. Ahti also appears to have some sort of knowledge of the Foundation, as it is where he led Jesse to give her his cassette player so she could navigate the Ashtray Maze.
A "City" is located elsewhere in the same Threshold, which the FBC has attempted to explore at least once. In the Quarry Incident, a safety inspector discovered all Quarry staff staring towards it in a trance-like state which lasted several minutes. Shorter working shifts and weekly psychological evaluations were recommended in the aftermath.