The Basement was seen in "Creepy Doll". It was seen when Mordecai, Rigby, and Pops were trying to find Percy. It was also seen in "Meat Your Maker". The Meat Locker was a part of the basement. It is the basement of Pops' house. In Creepy Doll, it was big and empty.
It was red, and empty with a few boxes laying in the corner. There are wooden stairs leading down there, and has a couple of tools.