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Salt mine, Aperture Science Enrichment Center
The Enrichment Shafts or Test Shafts, is a series of nine shafts located in the lowest levels (lower than 4375 m) of the Aperture Science Enrichment Center, in the salt mine originally bought by Cave Johnson in 1944, and used by Aperture Science for testing until around 1986.
So far, the shafts visited are Test Shaft 09 and a second unidentified Test Shaft. Test Shaft 09 is visited in the sixth and seventh chapters of Portal 2’s single-player mode, The Fall and The Reunion, when Chell is forced to navigate through Test Shaft 09 with GLaDOS powered by a potato, after Wheatley sends them there. Test Shaft 09, also nicknamed "Zulu Bunsen", was used to test the Mobility Gels. The second Test Shaft is visited in the fifth chapter of Portal 2’s co-op mode, Mobility Gels, when ATLAS and P-body navigate through the shaft's Enrichment Spheres to reach and open the Human Vault. This shaft was also used to test the Mobility Gels, but in cooperation mode between two Test Subjects, and minus the Conversion Gel.
The construction of the known shafts are of a completely different style and level of technology than that of the modular, panel-dominated space of the modern facility. The shafts have never been monitored by GLaDOS since they date from before her activation; consequently, pre-recorded messages from Aperture CEO Cave Johnson and his assistant Caroline were used to guide Test Subjects at least in Test Shaft 09. After GLaDOS' reactivation, she guides the robots in the second shaft.
As seen on the "You are here" image of the Enrichment Shaft series in Test Shaft 09, the shafts are typically made of 9 "Enrichment Spheres", "Geodomes" or "science spheres", where testing on various Aperture Science products was made in several smaller Test Chambers, closed, open, or semi-open to their surroundings, and numbered like the modern chambers. Test Shaft 09 however comprises only 7 spheres, while the co-op shaft comprises 11.
As seen in Test Shaft 09 again, a large elevator to be used by Test Subjects sometimes goes from the lowest levels to the surface.
The Enrichment Shafts appear to be connected by an underground train network, as Test Shaft 09 is seen to be connected to the other 8 shafts that way.
 Behind the scenes
- The Enrichment Shafts are what remains of the canceled game F-STOP, which originally was to be a prequel to Portal set during Cave Johnson's era, hence the old-fashioned design of the area. It was not to feature portals, and Johnson was to be a supporter later turning adversary, a role adapted for Wheatley in the final game.
- The last sign seen during Chell and GLaDOS' fall to Test Shaft 09 indicates 4375 m; as seen when Chell wakes up, the very bottom is lower than that. However, when adding up the different heights featured in Test Shaft 09 (the depth on top of the 6th sphere being 2975 m, a sphere having ~87 m in height, the first sphere having around a sphere's height under it, having ~50 m between the 50s and 70s spheres, and ~25 m between the 70s and 80s), a total of ~3660 m only is found; thus one of the two depths is incorrect.
- The game files reveal several logos, signs and posters not appearing in-game. It is unknown where exactly they were to appear, as they are remains of F-STOP, which would feature differences unknown so far. They are listed in the gallery below by decade. One of them refers to a "Geothermal Power Wing", in which it is stated there is nothing of interest. As most of the logos and signs found in the Enrichment Shafts, the texture file names start with "underground". However some textures with "underground" as a prefix are also found for instance in Turret Manufacturing, thus some of them may have originally made for other underground areas.