Bleach's Soul Society is a resting place of the dead. However, it's not the resting place. There is also Hueco Mundo, Spanish for "hollow world," where the restless dead go. Hueco Mundo reflects what a typical Hollow's life is like. It is barren and endless, and there is no salvation or paradise to be found. There is only one game on Hueco Mundo: kill, or be killed. Ichigo and his allies realized that when they stormed the place to find Orihime Inoue, and characters like Grimmjow Jaegerjaques and Barragan Luisenbarn realized for themselves that there is no victory here.
Hueco Mundo's Topography & Features
Hueco Mundo is not, as Ichigo once imagined, a murky swamp or a steaming jungle like the Amazon Rainforest. This is a place of the restless dead, the Hollows who have nothing but hunger and desperation in their lives. As a result, Hueco Mundo is equally barren, being nothing but a desert of white sands, rocky formations and odd, crystalline trees. As far as Bleach's characters can tell, this desert never ends, beneath an unmoving moon in a starless sky. Few clouds can be seen, but no rain has ever been observed. Strong winds can stir up sand storms, and some inhabitants of Hueco Mundo wear capes and cloaks to protect themselves from those storms. The spiritual energy in Hueco Mundo is fairly strong, being sufficient to nourish small Hollows without them needing to feed on other Hollows to survive. This bolstered Uryu Ishida's Quincy powers, though Arrancars also benefitted from this spiritual energy aura.
One notable location in Hueco Mundo is the Forest of Menos. It's a dense formation of trees underneath Hueco Mundo's surface where many Gillians reside, and a number of Adjuchas-class Hollows can be found there, too. The most noteworthy location is the massive fortress-city of Las Noches, which served as Sosuke Aizen's base of operations. It's unfathomably vast, a hollow space where many smaller buildings have been set up. The ceiling has an artificial sun, which allows Aizen to monitor the entire palace.
For transportation, most Hollows in Hueco Mundo simply have to run, fly or slither anywhere they want to go, since this world has no infrastructure outside Las Noches. Getting to and from this world is easy, however, since a Hollow or Arrancar can create a garganta portal to visit the world of the living at will. This garganta passes through the dangai, the great barrier that separates Hueco Mundo from the other two major realms.
Hueco Mundo In Bleach's Story
This world was mentioned and glimpsed a few times in the early story arcs of Bleach, but became significant in the Arrancar arc, after Rukia was rescued and Ichigo was working on mastering his inner Hollow. Sosuke Aizen had Hueco Mundo as his base of operations, and all of his Arrancar minions were stationed there, ready to carry out his orders. Chronologically, the story of Hueco Mundo began with the Vasto Lorde Barragan Luisenbarn, who conquered the entire world and united all of its Hollows into an army. He even considered the sky itself the ceiling of his castle, showing his omnipotent he was. But even this incredible accomplishment was ultimately futile, since Barragan was simply bored by this point. He had "won" the game or survival in Hueco Mundo, and it brought him no joy. He was knocked down a peg when Sosuke Aizen arrived and took over, and Barrgan became Espada #2.
Meanwhile, Grimmjow Jeagerjaques had a similar experience in this world. He was a panther-like Adjuchas, roaming the dunes until he met a small pack of Gillians. He was adopted as their leader and "king," and they aspired to become Vasto Lordes. It was a futile quest, just like Barragan's, and Grimmjow would simply eat everyone who got in his way until he was all alone, a king of nothing. Hueco Mundo became Hell for Hollows: it's miserable and violent with no salvation.
The Wandenreich, the Quincy empire, muscled in when Aizen was toppled and his grand plan foiled. Yhwach's army slaughtered many Hollows there, and the reigning queen of Hueco Mundo, Tier Halibel, was defeated and captured. Las Noches had been reduced to ruins. The Quincy Jagdarmee prepared to kill off the surviving Arrancars and use this place as a staging point for invading the Soul Society and other worlds. Once the Wandenreich was defeated, it can be presumed that Hueco Mundo returned to its natural state of affairs: a desolate, hopeless realm where Hollows try in vain to find salvation or "win" the game of survival.