Observation of a shadow of the Moon in the underground muon flux in the Soudan 2 detector

A shadow of the Moon, with a statistical significance of $5\ensuremath{\sigma},$ has been observed in the underground muon flux at a depth of 2090 mwe using the Soudan 2 detector. The angular resolution of the detector is well described by a Gaussian with a sigma $<~0.3\ifmmode^\circ\else\textdegree\fi{}.$ The position of the shadow confirms that the alignment of the detector is known to better than $0.15\ifmmode^\circ\else\textdegree\fi{}$ and has remained stable during ten years of data taking.