Moon

The is a planetary body floating above the world. The moon was created by the first jinchūriki, the Sage of the Six Paths, through his Chibaku Tensei technique. The Sage collected a massive amount of earth to seal the physical remains of the Ten-Tails after he used his Creation of All Things ability to divide its chakra into the nine tailed beasts, and then launched the stone prison into space, thus creating the moon.

Tobi's ultimate plan, the Eye of the Moon Plan, involves recreating the Ten-Tails through the other tailed beasts, become its jinchūriki, and then project his Sharingan onto the moon to cast his "Infinite Tsukuyomi": a worldwide-scale and perpetual genjutsu which will allow Tobi to control every living being on the planet, uniting them under him and creating a world with no war nor hatred, and making himself the ruler of the world.

Trivia

 * According to Gaara, because of its innate rage and bloodlust, Shukaku relishes the opportunity to kill anyone in sight, even more so during a full moon. This may be attributed to the Ten-Tails' presence within the moon.