Mystery of the Varna Gold: What Caused These Ancient Societies to Disappear?

Treasure found in prehistoric graves in Bulgaria is the first evidence of social hierarchy, but no one knows what caused the civilization's decline

from Travel | Smithsonian http://www.smithsonianmag.com/travel/varna-bulgaria-gold-graves-social-hierarchy-prehistoric-archaelogy-smithsonian-journeys-travel-quarterly-180958733/