Hueco Mundo

Hueco Mundo (虚圏 (ウェコムンド), Wekomundo; Spanish for "Hollow World", Japanese for "Hollow Sphere") is a world featured in Kingdom Hearts: Arcana. It is the dimension where Hollows and Arrancar usually reside.

Setting
The landscape of Hueco Mundo is a seemingly never-ending white desert, with dunes like those of the Sahara. The desert is littered with what appear to be boulders and shrivelled trees, but these are actually a form of quartz. In Hueco Mundo, the moon is on the opposite lunar phase of that in the Human World. The vast desert also seems to lack any bodies of water. Like in Soul Society, the atmosphere is filled with Reiryoku in the form of Reishi, but it is more concentrated, thus allowing smaller Hollows to gain nourishment despite the lack of Human souls.

Las Noches
Las Noches (虚夜宮(ラス・ノーチェス), Rasu Nōchesu; Spanish for "The Nights", Japanese for "Hollow Night Palace") is a massive fortress which can be seen for miles in Hueco Mundo. It is easily the largest building in the Kingdom Hearts universe, appearing as large as, if not larger than, Seireitei in Soul Society, capable of dwarfing even. It consists of a main building with a domed center surrounded by several large towers and smaller buildings. Atop the dome are six smaller towers (or as they appear), which seem to contain areas such as Aizen's throne room and the Espada meeting hall. Also, in the room where Orihime Inoue is kept, there is one large Middle-Eastern rug on the ground that is facing outside, much like those in rooms used for Islamic prayer. Like everything else in Hueco Mundo, the main building is colored white.