Da Vinci continued to work on this portrait for over a decade, possibly until his death in 1519. Its artistic importance lies in his innovative use of sfumato, a technique that creates a hazy, almost dreamlike quality through subtle transitions between light and shadow. The subject's enigmatic smile and the ambiguous background landscape, featuring a hazy valley and distant waterways, have also generated much discussion and analysis of the painting. Its unique position in popular culture stems mostly from its history. Its theft in 1911 thrust it into the global spotlight and cemented its status as a national treasure of France, and numerous reproductions, parodies, and references in popular media and culture have further contributed to its enduring fame. Considered the world's most recognisable painting, the Mona Lisa attracts around ten million visitors per year.