Most of Spain and all of Portugal were hit by a major blackout that impacted public transport, airports and phone service, leaving authorities scrambling to find the causes for one of the worst outages in Europe in years.

The outage, which hit millions of people, was due to sharp swings in power on the network, known as oscillations, according to grid operator Red Electrica. An excess of solar generation in the grid could have contributed to the incident. Spain has reported an unprecedented number of hours with negative power prices in recent months as more solar and wind power gets injected into the network.