Alanna Gladstone, a 40-year-old film editor, had landed in Portugal’s capital, Lisbon, on a flight from New York hours before the outage. She checked into her Airbnb and took a nap; by the time she woke up, the technology that the country takes for granted had gone dark.

The cause of Spain and Portugal’s widespread blackouts remained a mystery on Tuesday, with some isolated disruption remaining after power was largely restored to both countries.

A massive and unprecedented power outage on Monday hit large parts of Spain as well as Portugal. The blackout halted subway and railway trains, cut phone service and shut down traffic lights and ATMs for the millions of people who live across Europe's Iberian Peninsula.