Legendary Italian horror director Mario Bava assisted with the making of the special effects on this film. Bava passed away shortly before its release.
According to Irene Miracle, almost all her scenes were directed by Mario Bava. Dario Argento was rarely on set through most of the shoot.
Part of the reason Argento cast Irene Miracle as Rose Elliot was she had synchronize swimming skills, which came in quite handy for the shooting of the underwater ballroom scene.
Mario Bava performed a number of uncredited roles on this film. Bava was uncredited for the following duties: camera operator, lighting technician, visual effects artist, second unit director and even acting as a full director directing performances.