The CERN Neutrinos to Gran Sasso (CNGS) project aimed to unravel some of the mysteries surrounding neutrinos - light, neutral particles that hardly interact with matter. Three types or "flavours" of neutrino exist: the electron neutrino, the muon neutrino and the tau neutrino. But it seems that neutrinos are the chameleons of the particle world: they can change from one flavour into another. This phenomenon, called “oscillation”, occurs as neutrinos travel long distances through matter. The process is directly related to the neutrinos' tiny mass.

From July 2006 to December 2012, the CNGS project sent muon neutrinos from CERN to the Gran Sasso National Laboratory (LNGS), 732 kilometres away in Italy. Neutrinos interact so weakly with other particles that they pass easily through the intervening rock. At Gran Sasso, two experiments, OPERA and ICARUS, wait to find out if any of the muon neutrinos have transformed into tau neutrinos.

To create the neutrino beam, a beam of protons from the Super Proton Synchrotron at CERN was directed onto a graphite target. The collisions created particles called pions and kaons, which were fed into a system of two magnetic lenses that focused the particles into a parallel beam in the direction of Gran Sasso. The pions and kaons then decayed into muons and muon neutrinos in a 1-kilometre tunnel. At the end of the tunnel, a block of graphite and metal 18 metres thick absorbed protons as well as pions and kaons that did not decay. Muons were stopped by the rock beyond, but the muon neutrinos remained to streak through the rock on their journey to Italy.