Echidnaopolis is a stupid underground city based off an area in the Sonic Archie comics. It appears in both Julie-Su backstories and is mentioned in Lippies V.3 (and possibly the earlier versions but I don't remember).
Echidnaopolis is inhabited by primarily echidnas (entirely echidnas in the first version of the Julie-Su story). The area is isolated from society since the elevator is broken, as well as will not function from the underground panel. Therefore anyone who decides to venture down the elevator will not make it back out. Characters that became trapped in the city include Mighty, Bark and Bean. The most prominent characters that are found in the underground city are Googoo, Julie-Su and Lara-Le.