Valley of Thracian Kings keeps its secrets in grassy mounds

In the fields of Bulgaria they are everywhere - hundreds of mounds like huge molehills concealing the gold-filled tombs of ancient kings who left no other trace of their rule.

Known as tumuli, the burial mounds are the only remnants of the Thracian civilisation that inhabited the Balkan peninsula from the 2nd millennium BC to the 3rd century AD.

The accidental discovery of a tomb in 1944 revealed that the earthen structures were in fact manmade and that the burial monuments hidden within contained intricately crafted treasures.