A nexus of swirling blue light contained in the Dream Chamber deep beneath Marcuria. April Ryan discovers it while she is following the Prophet through the caves and Catacombs. According to her, it sounds like millions of people's voices - their dreams. When April touches the Vortex there is a clear reaction - lightning jumps from the ground, and in Stark, Eingana reacts.
According to Gordon Halloway, now with the Guardian's perspective, dreams have always passed between the twin worlds. However, recently, dreams have been passing from Stark to Arcadia, but not the other way around. This change may coincide with the creation of the Dream Vortex, or with WATI's use of it if the Vortex already existed. Given that Damien Cavanaugh tells Zoë that DreamNet's purpose is to "record people's dreams", it may be that the physical location of DreamNet is in Arcadia.