The Undersphere refers to the islandesque land mass below a sphere, positioned over the Infinite Ocean. It generally follows the theming of the original sphere above it, but some Undersphere isles are drastically different, or do not even exist. Incredibly watery spheres, for example, do not have a visible landmass beneath them.

Near Underspheres in the independent quarters, there are a number of water-dwelling cultures that dominate the water space around the Undersphere isles. Due to the pressure of deep water, their societies instead form rings that hug the isles of Underspheres.