Abstract This paper summarizes the main findings of a comprehensive study of the underground siting of nuclear power plants carried out at the Swiss Federal Institute for Reactor Research. Main aim of the investigations made was to identify suitable siting variants and to evaluate the feasibility, the safety potential and the cost of the concept. Two of the layouts developed for the main siting alternatives — the rock cavity alternative and the pit siting — are briefly described. In these designs an accident mitigation system based on the pressure relief concept, meant to reduce the consequences for the public and the environment in the case of extreme hypothetical events, has been proposed and an evaluation of its performances has been made to quantify the achievable risk reduction. The safety potential — especially under accident conditions — of this type of siting and the possibility that an underground plant may be exposed to new accident initiating events have been evaluated. From the technical point of view, an underground nuclear power plant is considered feasible while the economic penalty associated with the underground siting lies between 11 and 15%, according to the considered alternative.