How the Large Hadron Collider is being repaired
Engineers at the Large Hadron Collider at Cern in Switzerland have begun work to repair and upgrade the world's most powerful particle accelerator.
Once the work is completed, it will have double the power it previously had.
Researchers expect the LHC to discover new particles that could lead to a new theory of sub-atomic physics.
Pallab Ghosh was given exclusive access to the underground tunnels where the repairs are being carried out.