The Isarco Underpass is the extreme southern part of the Brenner Base Tunnel, and is located north of the town of Fortezza, in the Province of Bolzano.

The design speed is 250 km/h; to the exclusion of the stretch at the station of Fortezza, where the little space available makes it necessary to reduce said speed to 200 km/h.

The most important works in the project are the natural tunnels, dug with traditional method, with double coating.
The Isarco Underpass is made possible by the technology of freezing for ground stabilization. This technology is combined to an intervention of land pre-treatment by jet-grouting, to reduce the permeability of soils and the speed of the water in the riverbed.
Compared to the original solution, which provided the repositioning of the river bed, this helps to minimize the environmental impacts on the areas affected by the work, eliminating the lowering of the aquifer.