Human voices are largely used as sound effects, such as engine roars and earthquake sounds.
After the first screening of the film, director Hayao Miyazaki said it was the first time he ever cried during the screening of his own movie.
Concerned that surround sound in movie theaters distracted the audience from paying attention to what was happening on the screen, Hayao Miyazaki ordered the film to have a monaural sound mix.
The earthquake and fire was the result of the 1923 Great Kanto Earthquake.
The film is based on a manga Miyazaki authored in 2009. It was Toshio Suzuki who suggested to Hayao Miyazaki to adapt the manga into a film, when Miyazaki was interested in making a sequel to Ponyo y el secreto de la sirenita (2008).