The Islamic State's latest propaganda video shows fighters smashing statues and artefacts that are thousands of years old in the Mosul Museum, Iraq. The destruction is shocking, but maybe it is not random.

Archaeologist Katharyn Hanson of the University of Pennsylvania in Philadelphia has examined the video and points out that valuable objects are missing. She says that despite what the IS fighters say, they are not destroying everything.

The missing objects will likely be sold for a healthy profit on the black market, using international crime networks. Just how much money ISIS generates for its military campaign from looted art is still debated. Some believe the sale of ancient art is a key revenue stream for the terrorist group. Others, including Hanson, argue that ISIS makes far more money from oil stolen from pipelines and ransoms paid for hostages. Either way, treasures are being lost forever.