|Specifics: For GB:TVG SV|
The Abyss is a term from a data file in Ghostbusters: The Video Game (Stylized Versions), which is what you see in the in-game Tobin's Spirit Guide as it appears in the basement of the Firehouse, not the abbreviated version that appears in your field guide.
According to Tobin's Spirit Guide
Many myths and religions reference planes of existence beyond out own physical measurable one. One in particular seems to be either a wellspring of, or an attractor for, spirits. I've come to refer to this dimension as the abyss. On the few occasions I've seen it bleed over into the material plane, localized physical reality became twisted and unrecognizable.
Recent studies in multidimensional reality seem to support Tobin's assertions. I've also had a first-hand encounter with an interdimensional cross-rip that allowed me to gaze directly into the abyss, cementing its existence in my mind.
Same that goes for the Muon traps goes for this: Don't look directly into the abyss.
The art page can be found in the New York City Public Library, during the "Welcome to the Gozerian Public Library" section. It is on the bookshelf in the left alcove of the area at the bottom of the very long staircase.
No P.K.E. Meter scan is required.