Humans are an energy-hungry species, and our current sources of power are not cutting it. Nuclear fusion, the process that fuels the sun, might offer the kind of clean, abundant energy we need—if only scientists can figure it out. The International Thermonuclear Experimental Reactor (ITER) is the biggest and most ambitious attempt yet to harness the energy produced by forcing two atoms to become one. The $25-billion experiment in Saint-Paul-lez-Durance, France, is a joint project of the European Union, China, India, Japan, South Korea, Russia and the U.S. Its ultimate goal is to do what no fusion experiment has done before: produce more heat than it consumes.