Over Under

The Over Under is an alternate dimension that looks exactly like the main reality.

History
After Milhouse got taken by a Demogorgon, Lisa went to the Over Under to find and rescue him. When she arrived, she and Milhouse got attacked by Demogorgons and ran away into the town, which looked like a parallel version of their Springfield. After Homer saved Lisa and Milhouse, the Simpson family all ended up moving to the Over Under.

Behind the Laughter
The Over Under is a parody of the Upside Down from .