  Muography is an investigation technique based on the detection of the
atmospheric muon flux' modification through matter. It has found lately
multiple applications in geosciences, archaelogy, and non invasive industrial
controls. Mostly known for its imaging capabilities, muography may be exploited
as well for monitoring purposes since the atmospheric muon flux is available
permanently. In this paper we present an interesting measurement performed in
the context of an archaelogical project called Arch\'emuons, on the
archaeological site of "Palais du Miroir" in Vienne, South of Lyon, France. We
installed a muon detector in an underground gallery within the foundations of
the building for the second half of 2023. The primary goal is to measure
details of those foundations which are largely not excavated yet. Meanwhile we
observed over more than 6 months long-term and short-term variations of the
muon rates since the start of the experiment, which seem to exhibit a
correlation with the rain accumulating on the free field just above the
gallery. We propose as an explanation for this behavior the retention of water
by the soil above the detector site.
