These were what concerned physicists as LEP — the Large Electron Positron Collider at CERN — began operation in July 1989. This was a ring of magnets deep underground, 27 km in length, which accelerated electrons in one direction and positrons in the other. At four points around the ring, the two counter-rotating beams were brought into head-on collision so that electron and positron could mutually annihilate.