We study the neutrino and muon fluxes in extra-terrestrial environments. In environments with negligible atmosphere, we find a strong reduction of these fluxes relative to their intensities on Earth. Medium-energy neutrinos (50~MeV $<E_\nu<100$~TeV) show particularly strong suppression, by as much as 10$^3$ even at rather shallow depths. Such environments represent an opportunity for greatly extending the physics reach of underground searches in fundamental physics, including searches for WIMP Dark Matter, neutrinoless double-beta decay, and the diffuse supernova neutrino background.