The Abyss is created from memories of whomever manifested it-in this case, Reiko Asigiri manifested a main location, the Kuromiya Hospital, from the most painful experiences in her life. Other ghosts have manifested their own locations, such as the Caravan Internet Cafe, the Shosei High School, and others. It should be noted that some rooms (that exist in the real world, but not in the manifested reality) lead to a deep chasm with purple or gray, clouds. The clouds, as well as the chasm, seem to swirl endlessly, and falling into the chasm results in the character's immediate death.
Also note that The Black Page drags its visitors into Reiko's Abyss. Once having entered it, a person either falls into a coma after answering a cell phone call or falls into it inmediately.
Interestingly enough, Mnemonic means "Aiding in the help of remembering memories".
You also get a nice experience when looking into the Mnemonic Abyss from "a certain character "