This article pertains to material only described in in-game lore.
MC: Underside cannot currently be visited.
The Underside is the literal bottom side of the landmass Marianne's Magical Camp was built upon. Given that it is not used for training magical girls, it enjoys significantly more peace and quiet than the top half, though its lack of exposure to the false sun makes everything much darker here. Marianne allows this space to be populated by refugees from worlds ravaged by magico attacks, who have since formed at least one unnamed city. There is also a portal to Setra available here, though its exact whereabouts are currently unknown.
...Yes, this means that gravity in the Pocket Dimension is kind of screwy. Don't worry about it.