Heuco Mundo. Between the World of the Living and the Soul Society lies this vast wasteland. Dark and barren, with only the haunting light of a moon to show the way, Heuco Mundo is a depressing, hopeless mirror to the living world. When the living world was created, Hueco Mundo was produced as a byproduct, formed between the Living world and Soul Society to strengthen the barrier that divided the two. When a Hollow forms, it is here that they first appear, in the relative safety and spirit energy-rich environment of the realm they call home.
Geographically, Heuco Mundo is similar to the living world, with mountains, plains, steppes, forests, deserts, oceans, and nearly anything else you could think of, but merely a tainted perversion of the origional. The 'oceans' are a vile and acidic substance even the Menos can't tolerate, the forests are made of stone, and there's scarcely any real vegetation to be seen anywhere. Just about the only part of this realm that accurately resembles its living-world counterpart are the deserts, but even the deserts of the living world are more lively than those of Heuco Mundo. Without any real life to support, all the spiritual energy that would be infused with it instead permeates the atmosphere, sustaining lesser hollows between meals.
Within Heuco Mundo's dimensional space lays a pocket dimension, the 'Dangai Precipice.' This dimension takes the form of a massive tunnel that passes through Heuco Mundo between Soul Society and the Living World, serving as both a means of passage, and a pressure release valve between the two otherwise isolated dimensions. This passage is extremely dangerous, however, with indescribable 'things' in place to prevent unauthorized use. The only truly safe way to travel the precipice, is through the use of the Senkaimon, kido-based transport room, which serves as a safe crossing. One enters the room (which exists within the Dangai Precipice) from a dimension on one end, and exits it into the other, avoiding the Dangai precipice itself, cutting straight across Heuco Mundo, and entering the desired dimension.