The Standard Model of Particle Physics (the model of the strong and electroweak interactions) contains 3 types of neutrinos, all with zero mass: the electron, muon and tau neutrinos, associated with the electron, muon and tau lepton, respectively.
There are strong indications in favour of neutrino oscillations, that is for the possibility that one type of neutrino may transform into a neutrino of a different type. In this case one goes beyond the Standard Model and neutrinos have non zero masses.
Cosmic rays may yield muon and electron neutrinos: one speaks of atmospheric neutrinos, which originate in a spherical surface at about 20 km from the earth surface, and move rapidly towards the Earth.
1/2) Scheme of how atmospheric neutrinos originate.
An underground detector may measure neutrinos coming from below and from above.
MACRO has measured a number of atmospheric neutrinos smaller and with a different zenith angle distribution than anticipated, which are indications for neutrino oscillations. MACRO is also measuring high energy muons from above, and searches for rare particles in the penetrating cosmic radiation, in particular magnetic monopoles.
3) MACRO, located in the Gran Sasso Lab, has dimensions of 12 m x 10 m x 77 m.