We have observed underground cosmic-ray muons, corresponding to a secondary flux of \ensuremath{\sim}7\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}11}$ ${\mathrm{cm}}^{\ensuremath{-}2}$ ${\mathrm{s}}^{\ensuremath{-}1}$, at a depth of 1800 m water equivalent, which appear to be initiated by Cygnus X-3. This identification is based on both direction and phase coherence. The existence of such secondary muons conflicts with the current understanding of photon cascades and/or the nature and location of Cygnus X-3.