Hueco Mundo

Hueco Mundo (Spanish, lit. Hollow World) is the dimension where the Hollows and Arrancar reside.

Overview
Hueco Mundo is located between the human world and Soul Society, though it is separate from the bordering dimension known as the dangai. It can only be accessed by tearing the dimensional fabric separating the worlds, which allows 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 shriveled 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 Soul Society, the atmosphere is filled with spiritual energy, but it is more concentrated, thus allowing 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
Las Noches (虚夜宮, Spanish for The Nights) is the fortress and base of operations of Sōsuke Aizen, Gin Ichimaru and Kaname Tōsen after their defection from Soul Society. They reside there with their army of Arrancar.

Las Noches 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, every building is colored white.

Las Noches is easily the largest building structure in the Bleach universe, appearing as large as (if not larger than) Seireitei in Soul Society. Due to its immense size, Ichigo Kurosaki once commented that it could be an illusion as he, Chad, and Uryū Ishida did not seem to make any progress getting closer to it, despite running until they were exhausted. Also, according to the arrancar Nel Tu, it takes about three days to move around its width. Aaroniero Arruruerie claimed that the inside of the dome of Las Noches is sky blue, created by Aizen, so that the light of the sky would help him observe anywhere that he wishes. However, given that he was masquerading as Kaien Shiba at the time, this may only have been an excuse for him to stay out of the sun, and as such may not be accurate. The position of the halls within the palace can also be adjusted, allowing the one operating them to direct the movements of intruders.