made long underwater road tunnels possible, first used in the Holland Tunnel under the Hudson River. He began working under chief engineer Clifford Milburn Holland in 1915, and he finished directing construction of the Holland Tunnel after the death of Holland in the fall of 1924 and of Holland's successor Milton H. Freeman, who died in March 1925. Singstad also designed the Lincoln Tunnel, the Brooklyn–Battery Tunnel, and the Queens–Midtown Tunnel, the latter two as chief engineer of the New York City Tunnel Authority. In this capacity he clashed with Robert Moses, who not just preferred bridges but also sought to consolidate power and eliminate competition in civil engineering politics: In 1946, the Tunnel Authority was taken over by the Triborough Bridge Authority, forming the Triborough Bridge and Tunnel Authority, whereupon Singstad was fired, and the incomplete Brooklyn-Battery Tunnel was finished to specifications by TBTA chief engineer Ralph Smillie, which were mostly based on Singstad's original designs. This new design leaked, and the TBTA reverted to Singstad's original design. Singstad later claimed that Smillie had caused "excessive" leakage by not using Singstad's experimental caulking design to prevent leaks. Smillie denied that the leakage was excessive and that Singstad's caulking method had been replaced because that method was actually the cause of the leak. Other work Singstad was instrumental in numerous underwater vehicular tunnels worldwide. From 1930 to 1933, he designed and led construction of the Waasland tunnel under the Schelde River in Antwerp. The retreating French set explosives in the tunnel when the war began in 1940; the damage was repaired in weeks under German occupation. Later, the Germans tried again to explode the tunnel when they withdrew in 1944, and this time the tunnel was more seriously damaged. The tunnel was inundated with river water, and repairs took several years. He consulted on the Posey Tube, the second tube in the world, which used the same ventilation system that he had designed for the Holland Tunnel. He consulted on the Detroit–Windsor Tunnel, the third one, designed by fellow Norwegian-American engineer Søren Anton Thoresen, and designed the ventilation system. With Thoresen, he designed the Waasland tunnel under the river Scheldt in Antwerp, Belgium. On this project, Singstad designed the lining, the tunnel shield, the ventilation, and the equipment. He established the company Singstad and Kehart Consulting Engineers in 1945, which with 50 to 60 staff engineers engineered the final design