The sun beats down through a cloudless sky as we weave between concrete blocks, each about as tall a person. Hundreds of the blocks are arranged in lines that fan out from a central point, like a child's drawing of the sun, cast at the bottom of a huge limestone pit in southern France. It is as if I'm standing in a shrine to our closest star, and in a way I am. If all goes well, the space above my head will one day rage with humanity's first self-sustaining fusion reaction, an artificial sun ten times hotter than the one that gives our planet life.

The blocks - each fitted with an elastomeric top to absorb vibrations - are seismic plinths, designed to shield the building that will rise above from damage in the event of an earthquake. Together they form the bowels of ITER, the International Thermonuclear Experimental Reactor, an ambitious and unusual collaboration between seven of the world's biggest powers: China, the European Union, India, Japan, South Korea, Russia and the US. Their goal is to build the first energy-producing fusion reactor - harnessing the process that powers the sun and most other stars. At extremely high temperatures, hydrogen nuclei will fuse to form helium, spitting out more energy than the process consumes, something that has never yet been achieved by a human-made device.

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