Almost six million people in Southern California, Arizona, and Mexico found themselves without power Thursday after something went awry while an employee was carrying out a procedure at an Arizona substation, triggering a massive blackout.

San Diego, the eighth largest city in the US, saw its water supplies knocked out and all outbound flights from its airport grounded. Friday classes for local school children were canceled, as well.

“There was a very major outage, a region-wide outage,” said Mike Niggli, president of San Diego Gas and Electric. “There’s no doubt this has never happened before to our system.”

Late Thursday night, power was restored in about a dozen cities in San Diego and Orange counties, and nine of San Diego Gas & Electric’s 115 substations were back in service. Officials expect all those affected to have electricity back on sometime Friday.

The procedure that caused the problem in Arizona initially killed power to a line supplying electricity to Southern California before a domino effect swept across the Southwest.

Damon Gross, a spokesman for Arizona utility company APS, said, “There appears to be two failures here — one is human failure and the other is a system failure. Both of those will be addressed.”

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