This is the photograph by Jean-Pierre-Muller Javier Sorrian/AFP/Getty Images of the Louvre's Mona Lisa  and the copy housed at Madrid's Prado Museum. guardian

With the recent discovery in Munich of €1bn (£860m) worth of art looted by the Nazis, and the forthcoming release of a feature film, starring George Clooney, based on the exploits of the Monuments Men, it is a fitting time to recall how fortunate we are that so much art survived thesecond world war. The Nazi art theft division, the ERR (Einsatzstab Reichsleiter Rosenberg), was responsible for the theft of around 5m works: from the Louvre, the Uffizi and countless churches, galleries and homes. From headline-grabbing works like Michelangelo's Bruges Madonna to the most frequently stolen artwork in history, Jan van Eyck's Adoration of the Mystic Lamb, both of which feature in the Clooney film, to lesser-known gems that nevertheless held a place in the hearts of museumgoers or families, the story of art looting during the second world war is a tree with countless roots. Each masterpiece has its own history, a provenance ripe with intrigue. Few of the individual stories have been told, fewer still in depth.

Among the many enduring mysteries of this periodis the fate of the world's most famous painting. It seems that Leonardo da Vinci's Mona Lisa was among the paintings found in the Altaussee salt mine in the Austrian alps, which was converted by the Nazis into their secret stolen-art warehouse. 

The painting only "seems" to have been found there because contradictory information has come down through history, and the Mona Lisa is not mentioned in any wartime document, Nazi or allied, as having been in the mine. Whether it may have been at Altaussee was a question only raised when scholars examined the postwar Special Operations Executive report on the activities of Austrian double agents working for the allies to secure the mine. This report states that the team "saved such priceless objects as the Louvre's Mona Lisa". A second document, from an Austrian museum near Altaussee dated 12 December 1945, states that "the Mona Lisa from Paris" was among "80 wagons of art and cultural objects from across Europe" taken into the mine.