  A light Higgs portal scalar could be abundantly produced in the earth's
atmosphere and decay in large-volume neutrino detectors. We point out that the
Hyper-Kamiokande detector bears a strong discovery potential of probing such
particles in an uncharted parameter space that is actively explored by
intensity frontier experiments including rare kaon decays. The signal we
propose to look for is electron-positron pair creation that manifests as a
double-ring appearing from the same vertex. Most of these pairs originate from
zenith angles above the Hyper-K detector's horizon. This search can be
generalized to other new light states and is highly complementary to beam
experiments.
