Genbu

In the Land of Lightning, there is a gigantic and ancient turtle that Kumogakure has cared for since its founding. It serves as a training ground for the Kumo ninja, and is described as a moving fortress. Its size is so great that it can host other large animals as well as other natural and man-made structures. It is considered to be the equivalent of Konoha's Forest of Death (sans being built on top of a moving animal).

Background
A and Killer Bee trained on the island when they were younger. While there, Bee tamed and befriended all the animals on the island. He now has a vacation home there. When the Fourth Great Shinobi World War begins, the Shinobi Alliance sends Bee and Naruto Uzumaki, the only two remaining jinchūriki, to the island to be hidden from Akatsuki.

Appearance
The turtle's shell, the "island", has an accumulation of plant life that has grown on it. Despite being described by the Great Toad Sage as a paradise, it is anything but that to humans, with signs of death and battle everywhere and large, ferocious animals populating the island. The Falls of Truth can also be found on the island. The island is protected by a Giant Squid, which attacks anything that approaches, and a high-level barrier technique. In addition, the island can move, making it impossible to find without air reconnaissance. The turtle itself is one of the largest creatures in the series, if not the largest, easily surpassing the tailed beasts and the Sannin's summons in size.

It is mostly grey in colour, with spikes protruding from its shell and with a blue underbelly. Its shell is extremely sturdy, being able to withstand attacks of powerful jutsus and can even house the tailed beasts, though it is implied that its underbelly is less so, but still strong enough to handle lava release techniques and even one of Deidara's explosions, while receiving little damage.

Confining the Jinchūriki Arc
A number of Konoha ninja accompany Naruto as extra protection, including Might Guy, Aoba Yamashiro and Yamato, with Motoi serves as a guide for all of them. The official cover-story for Naruto's presence on the island is that he's "examining the island's ecology". Kisame Hoshigaki was also on the island, hiding inside the Samehada, which Bee has taken after their battle, until Naruto discovered him and forced him to escape. Motoi noted that if Kisame passes through the barrier, they will no longer be able to track him down. Though Kisame was ultimately forced to commit suicide, his messenger shark managed to take a scroll of information off the island to Madara, which resulted in the island needing to move to a safer location.

Later, Naruto is seen taking a "survey" of the islands giant animals, under the impression that it was part of his "mission". However Manda attacks the giant turtle's tail causing it to yell out, revealing its location to Kabuto Yakushi. While moving, the turtle has been intercepted by the Manda II, who bit its tail to prevent it from moving. After that, Kabuto ordered the newly-revived Deidara to implant a giant clay jellyfish bomb, which flipped the turtle upside down. The two Akatsuki allies and the Third Tsuchikage's party then landed on the belly-side of the turtle to do battle.

After the skirmish which ended up with Yamato being captured, Ōnoki carried the entire island turtle from the sea all the way back to Kumogakure.

Trivia

 * Due to the apes and the very obscure settings on the island, it might be a reference to Skull Island from the 1933 film King Kong. Killer Bee also affectionately calls an ape "King", a possible reference to the film's title character.
 * This also could be a reference to the World Turtle, a giant turtle or tortoise, supporting or containing the world on it's back. This myth appears in Hindu mythology, in Chinese mythology and in Native American mythology, notably the Iroquois.