In the autumn of 1925, when the Vatican hopes to achieve a modus vivendi with the Soviet Union had almost vanished, the situation of the Catholic Church in the Communist country was becoming increasingly dramatic and the concordat with Poland had introduced profound changes in the ecclesiastical structure of the frontier territories, the time seemed ripe for the realisation of a project pope Pius XI had reflected about since 1924, that foresaw passes aimed at alleviating the situation of the sorely afflicted Catholics in the Soviet Union by means that already evaded the official instances of the state or even proceeded in a completely clandestine way. Thus the known secret mission of Father Michele d´Herbigny SJ was born, i.e. the first tentative of the Holy See in the modern times to create underground structures of the Church in a state programmaticaly hostile to religion. On the basis of documents conserved in the Historical Archives of the Section for the Relations with the States of the State Secretariat of the Holy See, the article reconstructs the Vatican decision-making processes that preceded the mission and the criteria for the nomination of the new bishops and the apostolic administrators and for the territorial and jurisdictional reorganisation of the structures of the Catholic Church in the Soviet Union whose purpose was to adapt them to the new pastoral needs and the living conditions under the Communist regime that seemed destined to last a long time. In the second part, the paper analyzes the ways and means of communication (above all the various diplomatic channels) used by the Holy See to maintain contact with the particular Church in the Soviet Union, and the large charity actions launched by the Vatican to help people in need. The article focusses on the Catholics of German origin, who — as a result of the territorial changes after the war and the fall of the Russian Imperium, that caused a decrease of the quantitative force and changes in the ethnic composition of the Catholics in favour of the non-Poles — became a factor of extraordinary relevance in the framework of the Catholic Church in the Soviet Union.