The Beneathaverse appears in "Power Animal." It is the domain of the Gnome Ruler, as well as his minions. Finn is captured by the gnomes and taken down the well that lies just outside the Tree Fort. Finn is hooked up to various machines made to harness his extreme amount of power so the gnomes can "flip the world" using the Upturnoverdrive, making their kingdom on top of the world. It is unclear how broad the Beneathaverse is, but if the gnomes planned on flipping the whole Land of Ooo so the Beneathaverse would be on top, then it is possibly quite great.
Because it is underground, the kingdom is dark and rocky, and it seems that the only light comes from the windows in the cave. There are a few houses attached to the cave's walls. The Upturnoverdrive is located in their underground city.