The country’s top officials are trying to figure out the cause of a power outage that stranded tens of millions of people across the Iberian Peninsula.

A cross-border power blackout on Monday paralysed the Iberian Peninsula for most of the day, disrupting millions of lives across Spain and Portugal – even interrupting the French electrical system. The sweeping power outage that hit Spain and Portugal this week has raised questions about the electricity grid in a region not normally known for blackouts.

One of Europe’s most severe blackouts grounded flights, paralyzed metro systems, disrupted mobile communications and shut down ATMs across the Iberian Peninsula.