Spain-Portugal power outage leaves tens of millions in the dark. Could it happen in the US? - The U.S. electrical grids are vulnerable to multiple threats that could leave millions without power

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.

An unexplained blackout shut down airports, trains and traffic lights across the Iberian Peninsula on Monday, leaving tens of millions facing a night without electricity as officials raced to restore power.