For billions of years, magnetic fields have regulated the feedback
vital for returning matter to the interstellar and intergalactic
medium. However, the evolution, structure and origin of magnetism on the
largest scales are still open problems. I will review recent results
that showcase the unique view of magnetic fields provided by radio
astronomy. I will then explain how the Square Kilometre Array and other
future radio facilities can provide data that will reveal what cosmic
magnets look like, how they formed, and what role they have played in
the evolving Universe.