Power blackouts that left millions of people across Spain and Portugal without electricity may have been caused by a bizarre atmospheric phenomenon, though the true cause is yet to be determined.

Portugal’s grid operator Redes Energéticas Nacionais (REN) said electrical supply was lost across the entire Iberian Peninsula, and in parts of France, shortly after midday. Hours later, Spain’s Prime Minister Pedro Sanchez said authorities were still not sure what caused the blackout.