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Synopsis The Nathpa Jhakri Hydroelectric Project is world's one of the largest run-of-the-river scheme having 62.5m high concrete dam, 27.4 km long Head Race Tunnel (HRT) passing through high geothermal zones, the largest pressurised underground desilting chambers and the deepest surge shaft. This state-of-the-art project, constructed in the region of high mountain stresses and problems of geothermics with the expertise of GSI since inception to commissioning, is a matter of pride and admiration for the geoscientists. Commencing in late sixties, the geological investigations generated vast geotechnical data during preliminary, pre-construction and construction stages. The data, its geological implications and geotechnical solutions are scattered in different unpublished annual reports, study notes and technical memoranda issued from time to time. To disseminate and share their experiences gathered from 1967-2004 was viewed to bring out a comprehensive document encompassing each and every engineering geological and geotechnical aspect in the form of this write up. The write up provides detailed factual and analytical information on preliminary to construction stage investigation inputs during design stages of various components of this unique project. The unparallel problem of tunnelling through high temperature geothermal zones has been vividly discussed. This publication therefore is not only a testimony of the hard work done by the compiler but also a tribute to the GSI officers who have worked in the hazardous terrains and sacrificed their youth in the service of the nation.