Lured by the prospect of nearly inexhaustible source of clean energy, scientists have been investigating nuclear fusion reactors for decades, but a new facility taking shape in southern France will provide them with their biggest proving ground yet. ITER is set to become the world’s largest fusion device when completed in 2025, and has just moved into a vital phase with the assembly of the millions components now underway.

ITER, or the International Thermonuclear Experimental Reactor, has been in the works since 1985 and is one of the most ambitious energy projects humankind has ever undertaken. It is a collaborative endeavor involving thousands of scientists and engineers from 35 countries, all working to usher in a new era of renewable energy based on super-hot, high-speed reactions taking place inside the Sun.

These reactions see hydrogen nuclei smash into each under the forces of extreme heat and gravity, fusing together to form helium atoms and releasing monumental amounts of energy. Reactors like ITER are known as tokamaks, and seek to recreate these reactions inside donut-shaped chambers, where massive magnetic coils guide and compress streams of ultra-hot plasma to cause the hydrogen atoms to fuse.