A mysterious chamber deep under Marcuria, which is only accessible by traversing the caves and Catacombs. Once inside, there doesn't seem to be a clear entrance or exit. April Ryan enters the Chamber while on the trail of the Prophet, and finds the Chamber full of swirling blue light. The Chamber appears to be the repository and conduit for DreamNet in Arcadia, and is therefore possibly the site of some link between the two worlds.
When April returns to the surface, she tells Na'ane that the Chamber is directly below the Azadi Tower and, indeed, its outer wall was engraved with a petroglyph depicting it. As the Tower was built on the site of the Sentinel's ancient Temple of the Balance, the Chamber would have originally been below the Temple. Whether this is significant or not is not known, but April's first Shift ten years prior brought her straight to the Temple.