North Milford Valley, in western Utah, is home to dormant volcanoes, subterranean lava deposits, and smatterings of obsidian—black volcanic glass—that Paiute peoples once collected for arrowheads and jewelry. Scalding groundwater still bubbles to the surface in places. In such a landscape, you remember that the planet’s hard exterior, where we spend our entire lives, is so thin that we call it a crust. Its superheated interior, meanwhile, burns with an estimated forty-four trillion watts of powe...