The temple.
Located on the Island Turtle behind the Falls of Truth is a temple with many structures, headless Bodhisattva-like sculptures and murals depicting the Eight-Tails and the Two-Tails on a giant wall. Inside the temple there are two chambers where the jinchūriki attempts to gain control over their tailed beast.
The entrance to a training chamber.
The entrance to the first chamber where a jinchūriki attempts to control one's tailed beast has a sculpture of the Two-Tail's head with a switch inside its mouth, only accessible by entering head first. The chamber is designed to hold a tailed beast, in case the jinchūriki failed to master control and allow the beast to be reborn, and lock it up until a new jinchūriki comes forth.
The second chamber is known as the Chakra Isolation Chamber, and is located behind the statue of the Eight-Tails' head. It's seemingly impossible to detect chakra inside or from out of the chamber.