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I was shocked about "electric rocks" but it seems to be just a fancy word for "metallic minerals".
The article also says
Geiger determined that natural mineral deposits on the ocean floor, called polymetallic nodules, contained elements including cobalt, nickel, copper, lithium, and manganese—all of which are critical components of batteries.
I think I have seen some video about an Australian entrepreneur who wanted to "harvest" those things off the seafloor, to great concern of environmentalists.