Super Proton Synchrotron maintenance at CERN. Engineers working on inspection and maintenance during the Year-End Technical Stop for the Super Proton Synchrotron (SPS) accelerator at CERN, the European particle physics laboratory, Switzerland. The SPS was the first of the large accelerators at CERN, being an underground ring with a 7-kilometre circumference that extends across the border into France. It was completed in 1976. Magnets are used to control beams of particles as they are accelerated to high energies and smashed into targets or collided together. The SPS was used to discover the W and Z particles in 1983. It is now used to inject particles into even larger accelerators. Photographed on 22 February 2018.