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 reviving the Ten-Tails through joining the other tailed beasts, becoming its jinchūriki, and then projecting his Sharingan onto the moon in order to cast his "Infinite Tsukuyomi": a perpetual, world-scale genjutsu cast on every living being on the planet, which will allow Tobi to control and unite them to create a world with neither war nor hatred, making himself the ruler of the world.

Since Tobi began the revival of the Ten-Tails, the moon's mantle has began breaking off chunk by chunk and falling to the Earth.

Trivia

 * According to Gaara, Shukaku relishes the opportunity to kill anyone in sight at any time because of its innate rage and bloodlust, but especially so during the full moon. This may be because of the Ten-Tails' presence within the moon.