Ultra-high-energy (UHE) neutrinos , " , and " e at PeV and higher energies may induce air showers whose detectability is amplified millions to billions of times by their secondaries. We considered UHE -N and UHE " e -e interactions underneath mountains as a source of such horizontal amplified air showers. We also consider vertical upward UHE -N interactions (UPTAUs) on Earth's crust, leading to UHE air showers or interactions at the horizon edges (HORTAUs), and their beaming toward high mountain gamma, X-ray, and Cerenkov detectors, and we show their detectability. We notice that such rare upward air showers, UPTAUs and HORTAUs, may even hit nearby balloons or satellites and flash them with short diluted gamma bursts at the edge of the Compton Gamma Ray Observatory detection threshold. We suggest the possibility of identifying these events with recently discovered (BATSE) terrestrial gamma flashes (TGFs), and we argue for their probable UHE -UHE origin. From these data, approximated UHE fluxes and Dm l lower bounds are derived. Known X-ray, gamma, and TeV active Galactic and extragalactic sources have been identified in most TGF arrival directions. Maximal EGRET activity in the Galactic center overlaps with the maximal TGF flux. The UHE cosmic-ray (UHECR) Akino Giant Air Shower Array anisotropy at 10 18 eV also shows possible correlations with TGF events. The unique UHECR triplet in AGASA clustering, pointing toward BL Lac 1ES 0806+524, finds within its error box a corresponding TGF event, BATSE trigger 2444. Finally, a partial TGF Galactic signature, combined with the above correlations, suggests an astrophysical origin of TGF events.

The diffuse relic neutrinos with light mass are transparent to Ultrahigh energy (UHE) neutrinos at thousands EeV, born by photoproduction of pions by UHE protons on relic 2.73 K BBR radiation and originated in AGNs at cosmic distances. However these UHE νs may interact with those (mainly heaviest ν µr , ν τr and respective antineutrinos) clustered into HDM galactic halos. UHE photons or protons, secondaries of νν r scattering, might be the final observed signature of such high-energy chain reactions and may be responsible of the highest extragalactic cosmic-ray (CR) events. The chain-reactions conversion efficiency, ramifications and energetics are considered for the October 1991 CR event at 320 EeV observed by the Fly's Eye detector in Utah. These quantities seem compatible with the distance, direction and power (observed at MeV gamma energies) of the Seyfert galaxy MCG 8-11-11. The νν r interaction probability is favoured by at least three order of magnitude with respect to a direct ν scattering onto the Earth atmosphere. Therefore, it may better explain the extragalactic origin of the puzzling 320 EeV event, while offering indirect evidence of a hot dark galactic halo of light (i .e., m ν ∼ tens eV) neutrinos, probably of tau flavour.