With this paper, the correspondence of the renowned 20^th-century hydrobiologists and ecologists – V. I. Zhadin, M. M. Kozhov, and G. E. Hutchinson – is introduced for scientific use. These letters written in 1966-1968 touch upon the fate of the Lake Baikal in relation to the construction and putting into operation of the Baikal Pulp and Paper Mill on the banks of the lake and some other similar enterprises. This letter exchange was prompted by a number of articles published in The New York Times in 1966. The international aspect of media attention to the problem of the Lake Baikal conservation has not been covered in the history of science literature yet. The analysis of this correspondence is preceded by a description of the extensive efforts to protect the lake from industrial intervention, led by the renowned scientists, writers, and public figures. Particular attention is given to the speech of P. L. Kapitsa, Academician of the USSR Academy of Sciences, to the government bodies, in which he raised the issue of the long-term environmental consequences of polluting the lake with pulp and paper wastewaters. This correspondence is interesting in two aspects: as a significant milestone in the Russian and international environmental activities and as a demonstration of sociopolitical and ideological overtones in the work of the Soviet scientists. On the one hand, the letters discuss the reasons for keeping the lake clean (its endemic flora and fauna are important for the development of evolutionary theory and, therefore, according to Hutchinson, the death of the lake would be “not only an aesthetic but also a scientific disaster”) and, on the other hand, illustrate the situation in which the Soviet scientists existed at that time: they could not share with their foreign colleagues the information concerning many aspects of life in the USSR, including those related to nature conservation, let alone ask them for help.