H1 particle detector. Various layers of the H1 detector can be seen before they are closed together in position at the HERA collider at DESY, the German particle physics lab in Hamburg. The large grey cylinder is the 5.8 m diameter coil of H1's superconducting magnet. It will be moved to the left to line up with the beam pipe which passes through the small grey magnet visible at far left between the dull red concrete shielding blocks. The iron sections of the main magnet (blue), which moves in to surround the cylindrical coil, are layered with detectors to register particles such as muons. HERA collides electrons with protons in a 6.3km circumference tunnel.