CERN — the European Organisation for Nuclear Research — operates the world's largest particle physics laboratory on the Swiss-French border west of Geneva. The Large Hadron Collider, the most powerful particle accelerator ever built, runs in a circular tunnel 27 kilometres in circumference lying 100 metres underground beneath the fields here. It was in this tunnel, in 2012, that the Higgs boson was finally detected.
Free public visits are offered in two formats. The Globe of Science and Innovation, CERN's visitor centre, is open without reservation during regular hours and provides a good introduction to the work done here. Guided tours of the accelerator complex and experimental halls require advance booking (usually several weeks ahead) and vary in what they cover depending on which experiments are running. There is a long waiting list for the most popular tours. The tour that visits the actual underground tunnel is only offered when the accelerator is shut down for maintenance — check the calendar before planning around it. Take bus 18 from the city centre; CERN is a 20-minute ride from Cornavin station.