The SUBSPACE

The SUBSPACE is a place outside all AUs once created and it's very tiny if compared with their worlds, because the subspace is composed of a huge circle shaped plataform, doors cornering it, a fountain in the middle and a stairway to the Seraphim Shrine.

Backstory
After the events of Underverse, Ink Sans saw the necessity of creating a place to link all AUs in a safer way to allow quick travel. With that idea, he created the SUBSPACE and the Rifts that can be found in any AU, but very well hidden. These rifts are invisible and intangible, being necessary to cross one to know they are there, except in the Subspace, where the rifts are Door-shaped. These rifts not only link the AUs to the subspace, but link the AUs between each other, creating a network of gaps between the AUs, since they aren't completly separated, but linked between each other. Ink also created the Seraphim Shrine, home of the ultimate being, Seraphim Sans, the judge of everything and everyone.

The Doors
They are doors just like the ones from the ruins, but with a candle-like light on the front that is a signal of the integrity of that AU.

Light variations

 * Normal (lit)
 * Weak (fading)
 * Strong (blue fire lit)
 * Erased (unlit with locked door)

Crossovers
The SUBSPACE is not capable to link the UNDERTALE Multiverse with another franchises, being necessary another methods to reach another franchises. However the rifts are able to link those franchises, such as Freedom Planet and Chrono Trigger, but only directly, without having any connection to The SUBSPACE. Those rifts are more unstable and are in constant changes, sometimes leading to random places in time and space, besides being more rare and hard to find than the others that link the UNDERTALE AUs.