The Below is a dimensional matrix that was created as a prison by the Spaceborn. It was created from the moment the Spaceborn realized their spirit enemies would not fall lightly and would reform in the Above when torn apart.
The are 12 Dimensions intertwined within the Below that are connected via the 28 Rivers of the Below that form passages between them. Some of these dimensions, in a sense, lie deeper than other, from a higher dimensional perspective. Those lower dimensions are inhabitated by beings according to their power or danger posed to both the Above and Gaiga.
The first dimension of the below is attributed to all that failed the journey to the stars because of their evil inside them. So that the balance of the cosmos is not disrupted by them, they are send directly to that dimension to spend the rest of their conscious time there.
Dimensions (-02) -> (-08)
Those dimensions are attributed to the Horrors who were imprisoned by the Spaceborn for their crimes against Gaiga, the Above and reality as a whole. They are more torturous and dangerous to visit, as minions of chaos and Chaos Insects are to found there.
Dimensions (-09) -> (-12)
These dimensions serve as prisons of the Titans who destroyed both Gaiga and the Above and were banished therefore.