Asteroid M

Asteroid M was a large floating rock that served as the base for the mutant Magneto.

History
At some unknown point, Magneto formed Asteroid M. It is unknown whether this is an actual asteroid from space or simply a large rock and called it an asteroid.

Magneto offered Asteroid M as a sanctuary for opressed Mutants who would become his Acolytes. However, the whole thing turned sour due to Fabian Cortez betraying Magneto and framing the X-Men.