  Several proposed experiments will send beams of neutrinos through the Earth
along paths with a source-receiver distance of hundreds or thousands of
kilometers. Knowledge of the physical properties of the medium traversed by
these beams, in particular the density, will be necessary in order to properly
interpret the experimental data. Present geophysical knowledge allows the
average density along a path with a length of several thousand km to be
estimated with an accuracy of about $\pm 5$ per cent. Physicists planning
neutrino beam experiments should decide whether or not this level of
uncertainty is acceptable. If greater accuracy is required, intensive
geophysical research on the Earth structure along the beam path should be
conducted as part of the preparatory work on the experiments.
