  This paper describes precise measurements of the thermal neutron flux in the
LSM underground laboratory in proximity of the EDELWEISS-II dark matter search
experiment together with short measurements at various other locations.
Monitoring of the flux of thermal neutrons is accomplished using a mobile
detection system with low background proportional counter filled with $^3$He.
On average 75 neutrons per day are detected with a background level below 1
count per day (cpd). This provides a unique possibility of a day by day study
of variations of the neutron field in a deep underground site. The measured
average 4$\pi$ neutron flux per cm$^{2}$ in the proximity of EDELWEISS-II is
$\Phi_{MB}=3.57\pm0.05^{stat}\pm0.27^{syst}\times 10^{-6}$ neutrons/sec. We
report the first experimental observation that the point-to-point thermal
neutron flux at LSM varies by more than a factor two.
