Looting
Perhaps comprising as much as 75 percent of all art crime, antiquities looting is the most difficult crime to catch. Objects taken directly out of the earth or the sea will not appear on stolen art registries, because the objects never existed, at least not to the knowledge of contemporary society, before their illicit excavation.
The Getty Museum in Los Angeles decided to return the looted statue "Goddess, Probably Aphrodite" (425 – 400 B.C.) after complaints from the Italian government.
Photo by professor frenchie, courtesy flickr