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.

On Monday morning, millions of people across the Iberian Peninsula lost power. The cause of the outage is yet to be determined.

A failure of the interconnection between the grids of Spain and France caused the massive power outage that hit most of the Iberian Peninsula on Monday, La Vanguardia newspaper reported, quoting Spanish grid operator REE's system operations chief Eduardo Prieto.