At 8:50 p.m. on September 29, 2019, security guards at Arizona's Palo Verde Nuclear Generating Station noticed something unusual. A half dozen mysterious drones were flying over the facility's perimeter fence.

Housing three nuclear reactors, Palo Verde is the largest power plant by net generation in the U.S., supplying electricity to an area bounded by Texas on the east and California on the west. The plant is located just 50 miles (80 km) west of the city of Phoenix whose metropolitan area is home to almost 5 million people.

The security guards described the drones as being about two feet (60 cm) in diameter, flying at an altitude of between 200 and 300 feet (60 and 90 m), having red and white lights, and having employed their spotlights as they approached the plant.

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