Cosmic ray physics underground: some puzzles

A series of experiments has been carried out since 1961 at depths of 750 to 8400 hg/cm2 in the Kolar Gold Mines and the general features of atmospheric muons and neutrinos have been studied. However, there are some prolems which are incompletely understood and some of the events observed suggest the existence of new phenomena caused by unknown particles or processes. We point out some problems observed at these great depths which are relevant to the prompt muons in hadronic collisions; the increase of average transverse momentum in the very high energy region, muon bundles, Kolar events, large electromagnetic cascades, and so on