Cosmic rays research. View of one of the four telescopes which are installed on the Gran Sasso massif, Italy, at an altitude of 2017 metres. They are used to study the Cherenkov radiation produced by the interaction between the high-energy particles in the cosmic rays and atoms in the upper atmosphere. Cherenkov emission occurs because the particles in the cosmic rays have speeds greater than the speed of light in the atmosphere. The top of Gran Sasso also hosts the EAS-TOP experiment which detects some of the particles, such as protons and pions, which are present in the cosmic rays.