![View of the Large Electron Positron Collider with its monorail, the little electric train that allowed the technical teams to move around the machine.](https://cern70.cern/wp-content/uploads/2024/04/9107041_04-A4-at-144-dpi.jpg)
The Large Electron-Positron collider (LEP) is commissioned in July 1989. With its 27-km circumference, the LEP is the largest accelerator ever built. Fed by CERN’s accelerator complex, it provides particle collisions to four enormous detectors – ALEPH, DELPHI, L3, and OPAL.