Herat dates back to ancient times (its exact age remains unknown). During the period of the Achaemenid Empire (ca. 550-330 BC), the surrounding district was known as 𐏃𐎼𐎡𐎺 Haraiva (in Old Persian), and in classical sources the region was correspondingly known as Aria (Areia). In the Zoroastrian Avesta, the district is mentioned as Haroiva. The name of the district and its main town is derived from that of the chief river of the region, the Herey River (Old Dari Hereyrud, "Silken Water"), which traverses the district and passes some 5 km (3.1 mi) south of modern Herāt. Herey is mentioned in Sanskrit as yellow or golden color equivalent to Persian "Zard" meaning Gold (yellow). The naming of a region and its principal town after the main river is a common feature in this part of the world—compare the adjoining districts/rivers/towns of Arachosia and Bactria.