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.

Power was gradually restored for millions on Monday following what Spanish officials called a "rare atmospheric phenomenon."

The cause of the power outage is still unclear, although Portuguese Prime Minister Luis Montenegro says there’s “no indication” of a cyberattack.