Power had almost fully returned to Spain early Tuesday morning as many questions remained about what caused one of Europe’s most severe blackouts that grounded flights, paralyzed metro systems, disrupted mobile communications and shut down ATMs across Spain and Portugal.

Power had almost fully returned to Spain and Portugal on Tuesday although many questions remained about what caused one of Europe’s most severe blackouts

A blackout has brought much of Spain and Portugal to a standstill, affecting millions of people. The Spanish power distributor calls it “exceptional and extraordinary."