The flux of muons underground (above ~10 km water equivalent) are primarily produced in the atmosphere by cosmic rays. There is an abundance of literature detailing the flux, spectrum, and angular distribution as a function of depth. Though sparser, there are also measurements and some parameterizations of muon burst events, such as measured by the MACRO detector https://journals.aps.org/prd/pdf/10.1103/PhysRevD.56.1407 .
My question: are correlated multi-muon events observed underground primarily from
- multiple muons produced in the atmosphere by the same cosmic ray?
- showers produced in the rock by an isolated muon?
- something else?
