Electricity is pulsing through Spain and Portugal again after a massive outage knocked out power in both countries on Monday. So too are questions.

Millions of people in Spain, Portugal and parts of southern France lost power on Monday following a "severe disruption" in the European electrical system, the Spanish prime minister said. The cause of the disruption is still unknown,

Red Eléctrica, Spain’s electricity grid operator, ruled out a cyberattack, human error or unusual weather. Experts say it could take weeks to determine the cause.