View of LEP, the Large Electron-Positron collider at CERN, the European centre for particle physics near Geneva. LEP accelerates electrons & positrons to an energy of 50 GeV. It is housed in a circular tunnel 100m underground & 27km in circumference. The particles travel in opposite directions inside a narrow aluminium pipe which passes through large electromagnets. Test instruments sit on top of the magnets in this picture taken shortly before LEP started operations in July 1989. The long magnets are dipoles that bend the particles around the ring; the square blue magnets are quadrupoles that focus them in tight bunches. The particles collide at 4 points around the ring.