Longest particle accelerator tunnel

What to remember

Between France and Switzerland, near Geneva, CERN dug a 26.659 km ring for its Large Electron-Positron Collider, known as LEP. The bore was completed in 1988, and the circular gallery has a cross-section of 11.3 to 15.9 m². It runs roughly 100 m underground.

LEP operated from 1989 to 2000. It smashed electrons into positrons at energies that reached 209 GeV, making it the most powerful lepton accelerator ever built. Around 2001 the machine was dismantled to make way for the Large Hadron Collider, which re-used the same tunnel.

No particle accelerator tunnel is longer. Its looped layout sets it apart from road and rail tunnels: it links no two places and simply comes back on itself.