Geomagnetic storm
A disturbance of the Earth's magnetic field caused by a coronal mass ejection, able to induce destructive currents in large power grids.
In March 1989, a geomagnetic storm collapsed the Hydro-Québec grid in under two minutes. The reference event remains the one named after Carrington, in 1859. The mechanism is known: a solar coronal mass ejection reaches the Earth and induces parasitic direct currents in long conductors, extra-high-voltage transformers foremost. For insurance the peril is singular because it does not diversify by geography: it strikes all exposed regions as one. Its severity depends less on the number of transformers destroyed than on the time to replace them.
On July 23, 2012, a Carrington-class storm crossed the Earth's orbit. Earth missed it only because it was about nine days away from the trajectory.
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