The Kingdom of Aksum (Ge’ez: መንግሥተ አኵስም) was located in what is now northern Ethiopia, and spanned across modern-day Eritrea, eastern Sudan, Yemen, and southern Saudi Arabia. Emerging from the 1st century it started to decline by the 7th century until it disappeared in the mid-10th century. At its peak, the kingdom was considered one of the ancient world's four great powers alongside Persia, Rome, and China. Kings of Aksum started to mint coins (gold, silver and bronze) by the end of the 3rd century until the end of the 7th century, making it the first sub-Saharan African state to mint its own coinage and the last one to issue coins until the 10th century. Although no exact relationship is known between both systems nor between the aksumite coins within its own system, it is currently thought that the Aksumite currency system was based on a barley grain weight standard and somewhat closely related to Roman coinage system. Despite its position as one of the foremost and wealthiest empires of late antiquity, the Kingdom of Aksum is still relatively unknown and most of its history is still to be discovered; most of current knowledge being derived from unearthed coinage.