Dangai

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

Overview
The Dangai appears to be filled with bones and has the Kouryū and the as means of defence against unwanted intruders. The Dangai is cut-off from both the Living World and Soul Society in terms of time and space. The Janitor exists to stop anyone from lingering in the Dangai.

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, Uryū Ishida and Ichigo Kurosaki (attempting to save Uryū using his Zangetsu) were almost captured by the current. Luckily only his cape was caught, and Chad was able to break him free. Upon their escape to Soul Society, Orihime Inoue used her Santen Kesshun to block the Janitor, composed of the kouryū, and escape, but Yoruichi Shihōin berated her for this action; while the Kouryū only touched the shield area of her ability, had any one of Orihime's Shun Shun Rikka been touched by the current, the entire group would have been killed. If a Shinigami's Zanpakutō is used or revealed, they are also trapped by the current.

Janitor
The, 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--to assure that no one can loiter and take advantage of the time distortion. 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 its body. If the 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.

Shinigami aren't supposed to be able to harm the Janitor but Shūsuke Amagai using his Bakkōtō and Sōsuke Aizen have been able to destroy it.

Being chased by the Janitor has shown to lead to time distortion. Ichigo trained for 10 days in the living world, waited 7 days for the Senkaimon, and spent 5 days total afterwards before his 3-Day Bankai training. Logically speaking this is a total of 25 days, which is exactly (at the time) one day before Rukia's execution after Ichigo's training is complete. However, as luck would have it after being chased by the Janitor, Yoruichi and the Ryoka entered a twisted time-space tunnel when they escaped the Dangai, making their actual arrival to Soul Society about 7 days earlier. This was stated by Yoruichi in her thoughts on the first day of training, so this left a total of 2 days training left, and 8 days following the training that could possibly be used as an extension.

In addition, when Aizen destroyed the Janitor, Ichimaru commented that led to the axis slipping and resulted in them arriving in Soul Society a little further off from Karakura Town.

Plot Involvement
It was first used in order for Ichigo Kurosaki and his friends to invade Soul Society, through Urahara's Senkaimon. The Dangai was later shown in the Arrancar and Captain Amagai Arcs, by Ulquiorra Cifer to kidnap Orihime Inoue and the 3rd Division to kill a group of Menos Gillian respectively.

Later, Isshin opts to use the absence of the Janitor to their advantage by using the Dangai to teach Ichigo the Final Getsuga Tensho.

Appearance In other Media
The Dangai Precipice World was an integral plot-device for the first Bleach feature length film, Bleach: Memories of Nobody. It inadvertently spawned the Valley of Screams, and was revealed to play a role in the creation of Blanks.