Beneath the surface

The circular tunnel at CERN was built for a previous accelerator called the Large Electron and Positron collider (LEP). It's buried 100 metres below the surface - but why?

It was difficult to build underground, but cheaper than buying land on the surface. The rock surrounding the detectors protects them from outside influences such as the weather, and also absorbs dangerous radiation produced by the particle beams so that the radiation doesn't reach ground level.
 
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Watch building materials being carefully transported down the shaft to the ATLAS cavern.
Video: CERN
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