Bangladesh, Nov. 21 -- The Balkans have always been a treasure chest of ancient civilizations-Greek, Roman, Thracian, and countless local cultures that left behind millennia of heritage buried under fields, hillsides, and forgotten ruins. But for just as long, the region has been a hunting ground for criminal networks intent on digging up these treasures and selling them into the international art market. Now, one of the largest operations ever conducted against such trafficking has exposed a sprawling criminal network that quietly moved artifacts worth nearly a billion euros across Europe and beyond.

In what investigators describe as a historic crackdown, Bulgarian authorities-supported by Europol and coordinated by Eurojust-conducted a...