Parts of Spain, Portugal and southern France were plunged into darkness on Monday by a massive power outage. The lights are coming back on for some, but the cause of the outage remains unclear.

What caused the power outage? This is the crucial question that tens of millions of people in Spain and Portugal have been asking. And the answer is: We don’t know.

Power was almost entirely restored by Tuesday in Spain and Portugal, as well as in southern France, where households in the Basque region were affected. Here’s what you need to know: The cause ...

What we know about the Spain, Portugal power outages 01:47. Spain's government has set up a special commission to figure out what caused the massive power outage that hit virtually the entire ...

Parts of Spain, Portugal and southern France were plunged into darkness on Monday by a massive power outage. The lights are coming back on for some, but the cause of the outage remains unclear.