  Above a critical dark matter-nucleus scattering cross section any terrestrial
direct detection experiment loses sensitivity to dark matter, since the Earth
crust, atmosphere, and potential shielding layers start to block off the dark
matter particles. This critical cross section is commonly determined by
describing the average energy loss of the dark matter particles analytically.
However, this treatment overestimates the stopping power of the Earth crust.
Therefore the obtained bounds should be considered as conservative. We perform
Monte Carlo simulations to determine the precise value of the critical cross
section for various direct detection experiments and compare them to other dark
matter constraints in the low mass regime. In this region we find parameter
space where typical underground and surface detectors are completely blind to
dark matter. This "hole" in the parameter space can hardly be closed with an
increase in the detector exposure. Dedicated surface or high-altitude
experiments may be the only way to directly probe this part of the parameter
space.
