On 21 August 1911, when the Louvre was closed, Vincenzo Peruggia, an employee of the Louvre itself, entered the museum and stole the Mona Lisa
In this photo above you can see the empty space left in the Louvre in the days following the theft.
1911 is the year in which the first theft of a painting from a museum in history took place.
The painting involved was not just any painting, but the "Mona Lisa" (or "Gioconda") by Leonardo da Vinci.
On 21 August 1911, when the Louvre was closed, Vincenzo Peruggia, an employee of the Louvre itself, and therefore well aware of the security systems, entered the museum freely and without being noticed, detached the Mona Lisa from the wall, wrapped it in his jacket and left undisturbed. Peruggia returned home by bus, in the general indifference.
The thief kept the Mona Lisa hidden for about two years during which many important people of the time were interrogated (even Pablo Picasso was investigated and then acquitted). Even Peruggia was interrogated and his modest room was subjected to an inspection that had negative results as the Mona Lisa was hidden in a special space under the only table.
After about 28 months, he tried to resell it in Italy believing it was property of the Italian Nation. The authorities, through the testimony of one of the possible buyers, arrested Peruggia and retrieved the painting. In the photo we see the empty space left in the Louvre in the days following the theft of the Mona Lisa.