The Falls of Truth.
The Falls of Truth (真実の滝, Shinjutsu no Taki), located on an island in the Land of Lightning, is the place where Killer Bee trained to control the Eight-Tailed Giant Ox. It is a mysterious place where by sitting on the platform at the base of the falls, and concentrating the falls transport one into the inner world of the mind. Once there the falls becomes a mirror which reflects one's true self.
The entrance to a training chamber behind the falls
Behind the falls there is a place with many structures, headless Bodhisattva-like sculptures (which Killer Bee jokingly claimed were former jinchūriki who failed to control the Ox, in a manner similar to the toad statues of Mount Myōboku) and a picture depicting the Eight-Tails and the Two-Tailed Monster Cat on a giant wall in an unknown chamber. With Bee declaring this is where Naruto Uzumaki will fight and learn to control the Nine-Tailed Demon Fox, it can be presumed this is where Killer Bee and Yugito Nii also fought their tailed-beasts, the Eight-Tails and the Two-Tails respectively.
The entrance to a chamber where a jinchūriki attempts to control one's tailed beast has a sculpture of a demon head with a switch inside its mouth, only accessible by entering head first. Killer Bee also jokingly claimed that a jinchūriki with even a little trace of hatred will be decapitated by this sculpture. The chamber is designed to hold a tailed beast, in case the said jinchūriki failed to master control and allow the beast to be reborn, and lock it up until a new jinchūriki comes forth.