The recently completed Niagara Tunnel Facility Project in the province of Ontario in Canada is an extension of the Sir Adam Beck hydropower station originally built in the 19th century. The project includes the construction of a diversion tunnel to supply the existing hydropower station with an additional 500 m³ of water per second. The long-term stability of the unreinforced concrete lining is assured by a passive prestressed concrete lining according to the principles of Seeber. The operational water pressures reach 13 bar at the deepest point of the tunnel. In order to be able to monitor the slight deformations of the lining in the course of the prestressing process, an innovatively thought-out method of surveying had to be developed. The results of the deformation and long-term monitoring are presented.