The Underground, or UG for short, is an parallel plane of existence of Shibuya and the half of the Game that only Reapers and Players of the Game can see. It exists in the same space/time as the Realground (RG), but has a higher vibration frequency. Those in the RG cannot generally see, hear, or in any way perceive events and people in the UG, while those in the UG can perceive RG events freely (though physical interaction remains difficult). It is said in a report that the noise plane straddles both the UG and the RG.
Stray Noise created from people's negative emotions in the Realground are common in the UG, and can be fought by players through Scanning. The UG is the only place where Reapers and Players can use their powers and psychs. Certain designs (emblems) can connect the UG and RG, permitting Players to be seen by and interact with RG people and objects. The designs on a Reaper Creeper panel are an example of this, as are the emblems which permit Players to enter and use shops. The UG is highly tuned into the Noise Plane.
Joshua reveals that there are many UG's asides from Shibuya's, each one with its own set of rules. He compared this to how each human being's mind was separate and operated by its own set of rules.