Dangai Precipice World

The Dangai, or Dangai Precipice World to be specific (sometimes called "The Severed World"), is the dimension between Soul Society and the Human World.

Defenses
Naturally, the Dangai has some defenses against Hollows and other spiritual beings attempting to pass through the realm. This is why Shinigami are the only ones who can truly use it efficiently for travel between both the world of the living and the Soul Society.

Kouryū
Kouryū is the current that is constantly flowing through the Dangai, in order to prevent enemies such as Hollows from using it. The current stops souls from moving, and even one foot caught by it makes escape almost impossible. It encompasses the walls, capturing anyone who makes contact with them. Attempting to traverse the Dangai. Upon their escape to Soul Society, If a Shinigami's Zanpakutō is used or revealed, they are also trapped by the current.

Janitors
A Janitor (seimichiō; Cleaner in the English Dub), is the Dangai's equivalent to a street sweeper. The Janitor appears once every seven days to cleanse the area, killing anyone, and everything that makes contact with it. It looks like the same material making up the Precipice World, only moulded into the shape of a bullet train, and a golden eye at the upper-center of it's body. If a Janitor touches a Zanpakutō or other spiritual weapon, the user and weapon are trapped, since the Janitor's body is flowing with the Kouryū current. A Janitor can be "killed" with a sufficiently powerful attack, at least from a distance.

Time-Space Difference
Being an entirely separate area of space, the Dangai Precipice World has a completely different form of time than the human world, and Soul Society. Logically speaking this is a total of 25 days, which is exactly (at the time) 