Nakayo

The Nakayo (中世, Middle World), also called Middle Existence or to some the famed Limbo, is a dimension in between Soul Society and the Human World, and is accessible only through the Dangai.

Overview
The origins of the Nakayo are unknown. It was first discovered by Genryūsai Shigekuni Yamamoto after he wiped out the penal colony that once stayed in the Dangai, before the founding of the Gotei 13, though he could not access it; he only discovered it's entrance. The entrance to the Nakayo is found at the very end of the Dangai world, guarded by hordes of Kōtotsu (拘突, Wresting-Surge). The portal to this world is a long dimensional tunnel, at the end of the Dangai, made of the Dangai's Kōryū (拘流, Wresting Flow) which prevents Hollows and Shinigami alike from entering. The only way to enter the dimension, as shown by Raian Getsueikirite, is to subdue Kōtotsu with a special knife that he invented and to temporarily stop Kōryū with Kaikyō Kotei (界境固定, Realm Boundary Fixation) which opens the world up.

Raian describes the Nakayo as "Earth's own inner world", as the world is basically a barren version of the Human World. It possesses a city with towering buildings, though it is devoid of life. It possesses temples, but no one to run them. And lastly, it possesses luscious natural environments, but with no animal life to inhabit it. When Raian discovers this world, he makes it his army's headquarters, due to it's isolation and barren landscape.