Cosmic ray
Typical particles produced in such collisions are neutrons and charged mesons such as positive or negative pions and kaons. Some of these decay into muons. These are able to reach the surface of the Earth, and even penetrate for some distance into shallow mines. The muons can be easily detected by many types of particle detectors. The observation of a secondary shower of particles in different detectors at the same time shows that all of the particles came from that event. Several cosmic-ray observatories have been built to detect and study cosmic rays.