It is the Mona Lisa, or La Joconde, as it is called in French, that is Leonardo's most famous work. The painting was examined in detail in this exhibit.
The French engineer Pascal Cotte was given access to the painting, and his analysis of it indicates there's much hidden beneath the surface.
A replica of the painting was hanging, aside from the wall reproductions. That replica included the back of the painting (which I should have photographed), which is never seen except for curators. It includes some surprising details- writing and insignias. The replica, exactly on scale with the original, is smaller than you expect the painting to be.
The exhibit went into extensive detail about Cotte's analysis of the painting, and the portraits hidden beneath, including a draft that appeared to start it all. Cotte's conclusions aren't universally accepted, but certainly are intriguing. More from this tomorrow.