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The Mona Lisa is a famous painting by Leonardo da Vinci. It was painted in the year 1505 according to Mario's Time Machine and is a portrait of Lady Giocondo (noted by Michelangelo Buonarroti as having a captivating smile).
Mario is Missing!
In the DOS and Deluxe versions of Mario is Missing!, the Mona Lisa is stolen from its place in the Louvre by Koopa Troopas when they invade Paris. Its theft causes the Louvre to shut down, and Luigi is tasked with returning it. As he does not recognize the painting, once he obtains it, he asks the locals about it before bringing it to the museum. He then receives a reward of $2450, as well as a bonus of $2800 if it is returned first.
Mario's Time Machine
In the SNES and DOS versions of Mario's Time Machine, the Mona Lisa appears alongside Leonardo da Vinci. When Leonardo is first met, he refuses to talk, as he has lost his Notebook (which was stolen by a time-travelling Bowser) and he is unable to complete the painting without it. After Mario returns Leonardo's Notebook to him, he exclaims his thanks, saying that he is now able to finish painting the Mona Lisa. Mario then asks why the Mona Lisa is not smiling, to which Leonardo responds that she actually has a subtle smile, one that makes future generations wonder what she is smiling about.