Kamui's Dimension

is an alternate space that was created by Obito Uchiha's Mangekyō Sharingan. Kakashi Hatake's and Obito's Kamui share this same dimension when teleporting objects, due to the fact that each has one of Obito's eyes. This is further evidenced by the fact that objects warped by the former ability were still able to interact with Obito, despite his otherwise comprehensive "intangibility". Prior to learning the reason of this connection, Kakashi exploited it to injure Obito, still under the guise of Tobi, during the Fourth Shinobi World War.

Due to the two eyes being the only known pathways into and out of this dimension, there seems to be no other way of escaping it. With the destruction of both of Obito's eyes and his departure to the Pure Land, the path to this dimension is now sealed off unless someone uses the Summoning: Impure World Reincarnation to reincarnate Obito.

The dimension is hinted to be relatively close to Earth due to Obito having no problem using this technique multiple times a day but struggling to open Kaguya's far away dimension for even a few seconds.

Appearance
The location was first observed once Obito, as Tobi, transported both Sasuke Uchiha and Karin there after their battle at the Five Kage Summit, where it was shown to be composed from an innumerable amount of randomly arranged and differently sized rectangular prisms amongst a dark and seemingly endless void. Tobi has also apparently stored numerous giant shuriken as well as black receivers and gigantic kunai inside the dimension which he can launch at the opponent in quick succession.

Trivia

 * In Naruto Shippūden: Ultimate Ninja Impact, Obito has demonstrated the ability to manipulate the various box structures of this dimension to attack individuals trapped there.
 * In the anime, the box structures are shown to be constructed of earthen material, as Kakashi was able to use them as a medium for two earth techniques.