  Direct detection experiments relying on nuclear recoil signatures lose
sensitivity to sub-GeV dark matter for typical galactic velocities. This
sensitivity is recovered if there exists another source of flux with higher
momenta. Such an energetic flux of light dark matter could originate from the
decay of mesons produced in inelastic cosmic ray collisions. We compute this
novel production mechanism---a cosmic beam dump experiment---and estimate the
resulting limits from XENON1T and LZ. We find that the dark matter flux from
inelastic cosmic rays colliding with atmospheric nuclei can dominate over the
flux from elastic collisions with relic dark matter. The limits that we obtain
for hadrophilic scalar mediator models are competitive with those from
MiniBoone for light MeV-scale mediator masses.
