Most observers date the phenomenon to an Italian teen novel titled "I Want You, " published in 2006, featuring two Roman lovers who immortalized their bond on a bridge in the Eternal City and threw the key in the Tiber.

WSJ: Couples Adorn Bridges With Weighty Tokens of the Heart

Stretching north is the Golden Gate Bridge itself, saved from disaster by the heroics of Superman, James Bond and the X-Men, to name just a few.

BBC: The road to Hollywood, California