  Narrow muon bundles in underground detectors permit to study muoproduction
reactions that take place in the surrounding rock. We analyze the relevance of
a QED+QCD reaction, muoproduction of "open charm". The contribution to double
muon events is estimated to be 4-8 % of the one due to QED "trident" process,
for an ideal detector located under a rock depth of 3 km water equivalent, and
an observation threshold of 1 GeV.
