OPERA needs an intense and energetic beam of muon neutrinos traveling a distance of hundreds of kilometers to seek for the appearance of oscillated tau neutrinos. A beam of this type is generated from collisions of accelerated protons with a graphite target after focussing the particles produced (pions and kaons in particular) in the desired direction. The products of their decays, muons and neutrinos, continue to travel in generally the same direction as the parent particle. Muon neutrinos produced in this way at CERN cross the earth crust reaching OPERA after a 732 km journey. This facility (CNGS: “CERN Neutrinos to Gran Sasso”) has been built at CERN between 2000 and 2005 and it started operation in summer 2006. 3/12/2013 CNGS beam was stopped. A full description of CNGS can be found here.