Yuko Tani: “No way.”

Martin Lazzari: “I-Is this a city?”

Haruo Sakaki: “Why would such a...”

Rilu-elu Belu-be: “I-It's all Nanometal. But how? How did it get to such a large amount?”

Mulu-elu Galu-gu: “It took 20,000 years and kept growing. From Mechagodzilla's carcass. Yes, this is... Mechagodzilla City!”

― Yuko Tani, Martin Lazzari, Haruo Sakaki, Rilu-elu Belu-be, and Mulu-elu Galu-gu react to discovering Mechagodzilla City. (GODZILLA: City on the Edge of Battle)

The fourth distinct onscreen incarnation of Mechagodzilla, the anime Mechagodzilla was constructed in secret by the Bilusaludo and humans in Japan from 2042 to 2046, only to be lost in the final battle between the United Earth and Godzilla in March of 2046. However, Mechagodzilla's head remained intact, and its A.I. survived. Over the subsequent 20,000 years, Mechagodzilla's Nanometal gradually spread throughout the surrounding area, reconstructing its production facility around it and forming a massive Nanometal fortress dubbed Mechagodzilla City. When a deployment of United Earth soldiers returned to try and reclaim the Earth from Godzilla, they discovered Mechagodzilla City and prepared to use it to finally destroy Godzilla, who had grown to over 300 meters in height by this time. Godzilla was lured toward Mechagodzilla City and was attacked by the City's onslaught of weaponry, but Haruo Sakaki learned that the Bilusaludo intended to let Mechagodzilla's Nanometal spread across the planet and assimilate all life. He made the decision to stop Mechagodzilla City's expansion and destroyed its control room, allowing Godzilla to destroy it with two blasts of his atomic breath, eradicating every last trace of Mechagodzilla.

(and if your asking its not the city one, its the mech itself, this will take place in 2046 and a scenario if he actually activated.)