The hunt has begun for the cause of a massive blackout across Spain, Portugal and parts of France that halted trains, bank machines and traffic lights, in one of Europe's biggest ever power system collapses.

Electricity is pulsing through Spain and Portugal again after a massive outage knocked out power in both countries on Monday. So too are questions.

Massive Power Blackout Spain Portugal France has left millions without electricity, disrupting cities like Madrid, Lisbon, and parts of southern France. The blackout shut down airports, trains, metros,