The Vienna Police Dept. has a special unit that is assigned solely to patrol the city's intricate sewer system, as its network of interlocking tunnels make great hiding places for criminals on the run from the law, stolen property, drugs, etc. The "actors" playing police officers in the film were actually off-duty members of that unit.
A huge fan of the film, Martin Scorsese wrote a major thesis on it while in film school. He got a B+ for it, his tutor remarking "Forget it, it's just a thriller".
In Vienna the film has a permanent slot in one of its oldest cinemas. It is playing three times a week in the famous "Burgkino".
During meetings between Graham Greene and Sir Carol Reed with David O. Selznick, Greene was less than impressed with Selznick, who had (according to Selznick's own son) "become something of a parody of himself". Greene later mocked Selznick's dependency, at that stage, on the drug Benzedrine, better known as "speed". Reed also became hooked on Benzedrine while shooting the time-consuming film. Both Reed and Selznick were operating on as little as two hours of sleep a day.