For years developers have tried to figure out how to repurpose Kaiser Steel’s former open-pit iron mine at Eagle Mountain in Riverside County. One idea: Use it as a massive landfill, a proposal that fortunately never came to fruition. The current owners of the site now want to convert it into an immense, $2.5-billion hydroelectric battery, using daytime power to pump water from a lower-elevation pit to a pit 1,400 feet farther up the mountain, then running the water downhill at night through turbines to create energy.