The S1 Donau‐Lobau tunnel closes the gap in the road network around Vienna. Its two tunnel tubes run under the Danube and the Donau‐Auen National Park, with the provision that no work is allowed to take place on the surface of the site. The (hydro‐)geological subsurface conditions in the project area can be described as ”difficult“. The tunnel project must also meet all safety‐related requirements that arise during construction and operation, while at the same time minimizing the environmental impact.This article describes the essential special civil engineering and tunnelling measures in connection with the local subsoil conditions. It outlines the geotechnical exploration goals required for the structural tendering and detailed planning as well as for limiting the subsoil risk. Just obtaining the necessary permits for the exploration work required extensive preparatory work. As a result, both the exploration programme and the tunnel itself have been further adapted. The approved explorations were completed in the shortest possible time in compliance with extensive environmental requirements during a single winter season.