LOCATION: ASTEROID M
Asteroid M has been destroyed and reconstructed by the Master of Magnetism many times. All of them had several levels, including an observation deck, hangar bays and medical facilities. The various facilities had technology that kept it concealed from standard detection technology. They were all positioned in stationary orbit probably over Anya's grave (the deceased eldest child of Magneto).
Third version
It was during his brief association with the
Hellfire Club, that Magneto rebuilt a fancier version of Asteroid M. Magneto wanted a contingency plan, if things didn't work out with the Hellfire Club. Therefore, he proceeded to rebuild his base of operations, to survey the Earth, from high above. From there, he heard of Scarlet Witch's troubles. He went down to Earth, to bring her back to his space station. This version was first seen in
Avengers: West Coast #57 (April 1990).
When the West Coast Avengers attacked Asteroid M, we learn that Magneto has service robots on the station. The service robots were instrumental in alerting Magneto to the presence of
Wasp, on his base. For obvious reasons, his entire asteroid isn't made out of metallic objects.
Quicksilver demonstrated this when he ripped out plastic tubing out of one the consoles.
Afterwards events involving
Zaladane, in the
Savage Land, Magneto returned to Asteroid M, to live in peace. His peace didn't last that long, when
Acolytes, led originally by
Fabian Cortez, requested and were granted sanctuary by Magneto from American soldiers that were chasing them, on Asteroid M.
In
X-Men (Second series) #1 (October 1991), it was stated that the asteroid was in
synchronous orbit and 250 kilometers above the Earth (synchronous orbit is actually much farther from Earth than this; in all likelihood it was being
magnetically levitated at that altitude). The asteroid also has selective inhibitor fields, that may keep mutants or superhumans from using their powers. This version of the asteroid has medical & science facilities, an observation deck, sleeping quarters, and a swimming pool.
Asteroid M was destroyed for the final time in
X-Men (Second series) #3 (December 1991). From an escape pod, Cortez set off nuclear missiles that were around the asteroid. Although Magneto survived the re-entry of Asteroid M to Earth, the rest of the original Acolytes did not. The remains of the Asteroid M crashed into the
Middle East.
Forge,
Henry Peter Gyrich, and other government officials, inspected the wreckage in
Uncanny X-Men #299 (April 1993).