A widespread and unexplained power outage knocked out electricity in most of Spain and Portugal on Monday, shutting off traffic lights and causing chaos at airports, train stations and on the roads.

Power outages left tens of millions of people in Spain and Portugal in the dark on Monday. The blackouts shut off phone service and traffic lights, stopped trains, and cut off water access for U.S. Open champion Coco Gauff following her win at the Madrid Open, along with countless others.

Portugal’s grid operator Redes Energéticas Nacionais (REN) said electrical supply was lost across the entire Iberian Peninsula, and in parts of France, shortly after midday. Hours later, Spain’s Prime Minister Pedro Sanchez said authorities were still not sure what caused the blackout.