The River of Slime, also known as the Psychomagnotheric Slime Flow, is a flowing gigantic river comprised entirely of Mood Slime and it appears and talked about in great detail in Ghostbusters II. Later in Ghostbusters: The Video Game, the origins and purpose of the River of Slime are expanded upon.
It is suggested that the Mood Slime which the River of Slime is composed of, came from the negative emotions of all of the citizens of New York City, America's biggest city. It then materialized in a viscous psycho-reactive plasm with supernatural potential. It was last found in the abandoned subway New York Pneumatic Railroad (NYPR) flowing towards the Manhattan Museum of Art and it was used by the spirit of Vigo as the source of his powers.