A proton beam was taken from the SPS at and is made to collide with a graphite target within the CNGS tunnel. The resulting particles, most importantly kaons and pions among many other particles, were then focused by magnetic lensing and travelled down the CNGS tunnel in a vacuum tube. These particles are naturally unstable, and their decay products include muons and muon neutrinos. All particles except neutrinos (protons, muons, pion, kaon...) stop near the end of the tunnel. The neutrinos continue their flight unaffected, as they rarely interact with matter. The number of muons was measured at this point, which gave an indication of the beam's profile and intensity. This beam then passed through the crust of the Earth and it is expected that during flight some of the muon neutrinos convert into other neutrino types such as tau neutrinos. Once the beam arrived at Gran Sasso, the OPERA and ICARUS experiments were used to detect the neutrinos.