Where does the Kyuubi inside Naruto exist? Whenever Naruto sees the Kyuubi that Minato sealed inside him, we see this room with water on the floor and a big door behind which the Kyuubi stays. What is this place and where is it?
The place that we see is within Naruto's subconsciousness and its as a result of the Eight Tetragram pattern which is the sealing formula used by Minato to seal the Nine tails within Naruto.


  According to the Wiki, When Naruto wants to speak to the Nine-Tails, he enters a subconscious plane that resembles an underground complex, with pipes running along the ceiling and various hallways. The floor is flooded with a shallow layer of water; the Nine-Tails' growing influence over Naruto is sometimes depicted as the water level rising.