  Tau neutrinos are unique cosmic messengers, especially at extreme energies.
When they undergo a charged-current interaction, the short lifetime of the
produced tau gives rise to secondary tau neutrinos that carry a significant
fraction of the primary neutrino energy. This effect, known as tau neutrino
regeneration, has not been applied to its full potential in current generation
neutrino experiments. In this work, we present an updated calculation of tau
neutrino regeneration, and explore its implications for two scenarios: the
recent anomalous ANITA events and the cosmogenic neutrino flux. For the former,
we investigate the idea of localized emission and find that the maximum
secondary neutrino flux allowed by IceCube measurements implies a primary flux
that is incompatible with the ANITA observation, regardless of the assumed
source energy spectrum. For the latter, we study the prospect of detecting the
cosmogenic neutrino flux of regenerated PeV neutrinos with current and next
generation neutrino detectors.
