A small artificially created universe linked to the current universe by a wormhole. Also known as baby universes, in their natural form. According to various Archailectologists, these can theoretically be used for living space, computing or as an escape from a decaying universe, among other purposes.
Basement universes and the methods of their creation are very much in the realm of clarketech/ archailect godtech. Scholars of the archailects believe that most use basement universes to some form or another. Supposedly used as so-called Tipler Oracles when required, and possibly for other purposes which are not known to modosophonts.