the center of a planet.
earth has a molten nickel-iron core. Current fluctuations in the core probably cause earth's magnetic field.
david brin theorized in his novel earth that the core and mantel of the earth contained superconducting substrates. Add a few gravity waves to reorganize the thing and you get a giant, artificial intelligence- producing neural net inside the planet. very cool.
'specially when the earth gets pissed.