  We consider the upscattering of atmospheric neutrinos in the interior of the
Earth producing heavy neutral leptons (HNLs) which subsequently decay inside
large volume detectors (e.g. Super-Kamiokande or DUNE). We compute the flux of
upscattered HNLs arriving at a detector, and the resultant event rate of
visible decay products. Using Super-Kamiokande's atmospheric neutrino dataset
we find new leading constraints for dipole couplings to any flavor with HNL
masses between roughly 10 MeV and 100 MeV. For mass mixing with tau neutrinos,
we probe new parameter space near HNL masses of $\sim 20$ MeV with prospects
for substantial future improvements. We also discuss prospects at future
experiments such as DUNE, JUNO, and Hyper-Kamiokande.
