The cause of the power outage that severely disrupted life in Spain and Portugal on Monday remains a mystery as service was brought back to the Iberian Peninsula.

Power has been restored to tens of millions of people across almost all of Spain and Portugal, but disruptions to transport services were expected to stretch into a second day as questions mount over what caused a massive blackout.

Electricity was back in most of the two countries almost 18 hours after a blackout that shut down much of daily life. The cause of the outage remained unclear.