Spain and Portugal are racing the setting sun as they respond to a massive, unexplained power outage that knocked out traffic lights, caused chaos on roads and in airports, and threatened to plunge tens of millions of people into darkness on Monday.

Spanish power distributor Red Eléctrica said that restoring power fully to the country and neighboring Portugal could take 6-10 hours.

A countrywide power outage impacted Spain—as well as swaths of Portugal and smaller portions of France—Monday afternoon, affecting essential services and wreaking chaos across the Iberian Peninsula. The cause of the outage is currently being investigated by Spanish and Portuguese officials and ministers.