This cache is located on Bonneville Ave which was logically named after the Bonneville substation. Congress created BPA in 1937 to deliver and sell the power from Bonneville Dam.
Back when I was young, (yes, that was a long time ago), my neighbor told me that this was one of the largest substations built. He also said most of it was built under ground to protect it from being easily seen and attacked during the war. I don't know if any of the story was true but it stuck with me all of these years.
Have fun finding the cache.