When firefighters were called to an Arizona Public Service Co. facility in Surprise earlier this year for a fire, they didn't know there were giant batteries inside, storing energy for future use on the electrical grid.
They found out when they got to the scene, and conducted tests before entering the building.
After firefighters had entered, the place exploded. Four firefighters were injured.
As the investigation into the April 19 explosion continues, Valley cities are enacting new laws to try to make sure something similar doesn't happen again.
"Once you have an incident that hurts firefighters, it wakes us all up," said Brian Scholl, deputy fire marshal for the Phoenix Fire Department.
This summer, Phoenix, Peoria and Surprise enacted laws that for the first time address how giant batteries are stored. Mesa is considering doing the same.