In this talk, we discuss issues relevant to the detections of Earth-skimming/mountain-penetrating tau neutrinos. We first argue that there are non-negligible astrophysical tau neutrino fluxes due to neutrino flavor oscillations. We then discuss the rationale for detecting Earth-skimming and mountain-penetrating tau neutrinos. The \nu _\tau \to \tau conversion efficiencies and the tau-lepton event rate are presented.

PACS numbers: 95.85.Ry, 14.60.Fg, 14.60.Pq, 95.55.Vj