  For nearly a century, studying cosmic-ray air showers has driven progress in
our understanding of elementary particle physics. In this work, we revisit the
production of millicharged particles in these atmospheric showers and provide
new constraints for XENON1T and Super-Kamiokande and new sensitivity estimates
of current and future detectors, such as JUNO. We discuss distinct search
strategies, specifically studies of single-energy-deposition events, where one
electron in the detector receives a relatively large energy transfer, as well
as multiple-scattering events consisting of (at least) two relatively small
energy depositions. We demonstrate that these atmospheric search strategies --
especially this new, multiple-scattering signature -- provide significant room
for improvement in the next decade, in a way that is complementary to
anthropogenic, beam-based searches for MeV-GeV millicharged particles. Finally,
we also discuss the implementation of a Monte Carlo simulation for millicharged
particle detection in large-volume neutrino detectors, such as IceCube.
