  The natural neutron background influences the maximum achievable sensitivity
in most deep underground nuclear, astroparticle and double-beta decay physics
experiments. Reliable neutron flux numbers are an important ingredient in the
design of the shielding of new large-scale experiments as well as in the
analysis of experimental data.
  Using a portable setup of He-3 counters we measured the thermal neutron flux
at the Kimballton Underground Research Facility, the Soudan Underground
Laboratory, on the 4100 ft and the 4850 ft levels of the Sanford Underground
Research Facility, at the Waste Isolation Pilot Plant and at the Gran Sasso
National Laboratory. Absolute neutron fluxes at these laboratories are
presented.
