The magic of movies makes a huge scenic warehouse out of any city, and Venice, the scenic city par excellence, is no exception: in Indiana Jones and the Last Crusade (1989), Indy seeks the map to the Holy Grail in the basement of an old library, has a narrow escape as usual, and finally emerges from a manhole.
The library is in fact St. Barnabas deconsacrated Church, for years home to a (rather touristy) exhibition on Leonardo da Vinci, and the underground simply does not exist, since in Venice there’s nothing below street level, apart from drainpipes…