A muon (pronounced myoo·aan and named for the Greek letter mu (μ), which is used to represent it) is a naturally occurring sub-atomic particle produced by cosmic rays striking the Earth’s atmosphere. These rays are not harmful to us or any other life on the surface of Earth. Muons pass through the surface of the Earth on average about once per minute for every square centimetre. To make it more personal, a muon passes through your head once every second!