Earth, in the 2017 crossover episodes also refered to as Earth 7, is a rocky planet orbiting the Sun, and the third planet in the Solar System in order from the star it swings arounds. It is the home planet of the Ninja Turtles, and many of their their allies and antagonists. For millions of years is said to have been the site of a secret invasion of the alien race known as the Kraang, who seek to mutate Earth into a new homeworld with characteristics of their native reality, Dimension X.
Kraang Prime claims that the Kraang discovered Earth millions of years before the present day, and exposed early primates to Mutagen produced by Dimension X's Kraathatrogons. This supposedly led to the birth of the human race, whom the Kraang continued to interact with and exploit down through the ages and primarily, the Kraang sought to use humanity to perfect the Mutagen, which on Earth had different properties than the Kraang had expected. At some point they began utilizing robotic exo-suits known as Kraangdroids that possessed a human appearance-uniformly appearing as a dark-haired human male-developed by former Kraang turned Utrom Bishop.
Some centuries prior to the present day, the Foot Clan was founded in Japan by Koga Takuza, the feared ninja who forged the Kuro Kabuto. The Hamato Clan later arose to challenge the Foot Clan, and eventually succeeded in wiping most of them out, while Oroku Keiji's son Oroku Saki was taken in by the Hamato Clan and raised alongside Hamato Yoshi but however, the Hamato Clan began to decline, and Saki eventually learned of his true heritage. Reforming the Foot Clan, he left the Hamato Clan virtually destroyed except for Yoshi, and turned his attention towards building an international criminal empire.
The Kraang eventually focused their invasion efforts on the metropolis of New York City, then in the present day set about kidnapping scientists in preparation for their efforts to "Kraangform" the planet. They also set their sights on April O'Neil, a human girl whose DNA was the product of Kraang experiments on her family line intended to get them the strain of DNA needed to perfect the Mutagen for use on Earth. April also developed mental energy "uniquely attuned to [Earth's] universe," an ability that Kraang Prime intended to steal and use to transform the Earth. However, the Kraang's machinations soon met with interference from the Ninja Turtles, four mutants created by Kraang Mutagen and trained in the art of ninjitsu by Yoshi, who had relocated to New York and was mutated into a large humanoid rat.
The conflict between the Turtles and the Kraang-which the Foot Clan also became participants in-saw Earth undergo two full-scale invasions by the Kraang. The second saw New York occupied by the Kraang, despite the efforts of both the Turtles and the Earth Protection Force to stop them. However, this occupation was later overthrown by the efforts of the Turtles and the Mighty Mutanimals, and the Kraang were sent back to Dimension X.
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They subsequently returned, but were followed by their long-time foes, the Triceratons, who determined to destroy Earth to rid it of the Kraang. They eventually employed a black hole generator to accomplish this task, resulting in the apparent destruction of Earth. Out of it's billions of inhabitants, only six were known to have escaped: the Turtles, April, and Casey Jones, all of them having been rescued by the last-second arrival of Professor Zayton Honeycutt.