A "terrorist startup with a clearly defined business model" is how religious historian Karen Armstrong describes Islamic State. The radical Islamist group is the world’s richest terrorist organisation – those who have seen the group’s grotesque propaganda videos may notice fleets of brand-new 4x4s its members drive.

But where does the money come from? Analysis suggests donations, smuggled oil (up to $1.645m a day), kidnapping (at least $20m last year), people trafficking, extortion, robbery and last – but not least – the sale of antiquities. It’s a lucrative source of income – for example, the sale of looted items from al-Nabuk, west of Damascus, is reported to have earned IS $36m.

IS operates in the richest archaeological arena in the world, the cradle of civilisation. While ancient sites at Nimrud, Nineveh and Hatra are being destroyed, a stream of artefacts suspected to come from such places has appeared on the black market. IS either uses so-called ‘bulldozer archaeology’ (unearthing sites using any equipment available which is extraordinarily destructive), or employs locals to dig up sites and tombs. The group then takes a tax, approved by Sharia law, based on the value of any treasure taken. No-one knows what has come out of the ground and such loot is impossible to identify later.

Do not be fooled by the video of IS in Mosul Museum smashing ancient Assyrian statues which it claimed were "worthless idols". IS may have defaced important monuments, which it cannot sell, but evidence suggests it is trading in moveable objects, which it can. In any case, the statues in the museum were plaster copies. "None of the artefacts is an original,” says the head of Iraq's national antiquities department Fawzye al-Mahdi.