Hueco Mundo

is the dimension where Hollows and Arrancar usually reside.

Overview
Hueco Mundo is located between the Human world and Soul Society, though it is separate from the bordering dimension known as Dangai. It can only be accessed by tearing the dimensional fabric separating the worlds, allowing Hollows to travel to any other dimension at will. Depending on the Hollow, the effect of this process varies. Regular Hollows usually appear from a small black vortex or form out of smoke, while Menos-class Hollows quite literally tear the sky apart to emerge. Arrancar use a more stable variant of the latter technique known as Garganta.

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. 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.

Like that in real world deserts, active life in Hueco Mundo appears to be few and far between. Hollows generally seem to dwell below the surface or in other areas until something provokes them to leave their shelter, like the intrusion of a foreign entity. According to the Shinigami, all Hollows dwell in Hueco Mundo regardless of their shape, size, or power. In the anime, Hollows can also drag other spirits into Hueco Mundo, as seen with Orihime Inoue's brother, Sora. In this case, Sora was forcefully made into a Hollow by several others and sent against Ichigo Kurosaki.

Las Noches is also located in Hueco Mundo. Formerly ruled by Baraggan Luisenbarn, King of Hueco Mundo, it is now the headquarters of Aizen and his recently-acquired army of Arrancar.

As explained in an anime-only filler, there exists a vast forest underneath Hueco Mundo where Menos-class Hollows reside. The trees in this forest extend to the surface and dot the landscape above ground. The Adjuchas-class Menos which reside in this forest are under the control of Aizen.