The Underground, or UG, is a parallel plane of existence of Shibuya that only Reapers and Players of the Reapers' 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.  Certain people are able to see the UG or the Noise plane from the RG, what causes this being unclear. Joshua is the most prominent example, though certain random NPCs have scan dialogue indicating they are at least able to perceive the existence of Noise symbols or even Reaper wings while still living in 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.

Spoiler warning: Plot and/or ending details follow.

Joshua states that there are many UGs asides from Shibuya's, each one with its own set of rules. He compares this to how each human being's mind is separate and operates by its own set of rules.

Spoiler warning: Spoilers end here.

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