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.

A massive power cut hit large parts of Spain and Portugal on Monday, causing widespread chaos and disruption. The outage left millions of people without electricity, while internet and phone networks were also down.

The cause of the power outage is still unclear, although Portuguese Prime Minister Luis Montenegro says there’s “no indication” of a cyberattack.