Red Eléctrica, Spain’s electricity grid operator, ruled out a cyberattack, human error or unusual weather. Experts say it could take weeks to determine the cause.

One of Europe’s most severe blackouts grounded flights, paralyzed metro systems, disrupted mobile communications and shut down ATMs across the Iberian Peninsula.

BARCELONA, Spain (AP) — The blackout that engulfed the Iberian Peninsula on Monday was chaotic and stressful for many. But it had a silver lining: People were forced away from their screens and into the “real” world, connecting with the present and one another. Some Spaniards and Portuguese embraced their day without power.