Dangai

The Dangai, or Dangai Precipice World to be specific, is the dimension between Soul Society and the Human World. 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. It appears to have bones, scattered all across the floor, and some other more prominent skeletal remains while the entire dimension itself is dark, purple and gelatinous, moving like a creature.

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.

Defenses
Naturally, the Dangai has some defenses against Hollow 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, 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 Yasutora Sado was able to break him free. Upon their escape to Soul Society, Orihime Inoue used her Shanten Kisshun 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.

Janitors (Seimichiō): A Janitor, (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 molded 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. The only person shown defeating a Janitor is the anime-only captain, Shūsuke Amagai, using his zanpakutō's projectile ability (protecting himself from the current) to obliterate it. This implies that 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. For example, Ichigo trained for 10 days in the living world, waited 7 days for the Senkaimon, and spent 5 days total afterward 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.