The Transcaucasus region and Dagestan were the furthest points of Parthian and later Sasanian expansions, with areas to the north of the Greater Caucasus range practically impregnable. The mythological Mount Qaf, the world's highest mountain that ancient Iranian lore shrouded in mystery, was said to be situated in this region. The region is also one of the candidates for the location of Airyanem Vaejah, the apparent homeland of the Iranians of Zoroaster. In Middle Persian sources of the Sasanian era, the Caucasus range was referred to as Kaf Kof. The term resurfaced in Iranian tradition later on in a variant form when Ferdowsi, in his Shahnameh, referred to the Caucasus mountains as Kōh-i Kāf. Most of the modern names of the Caucasus originate from the Greek Kaukasos (Lat., Caucasus) and the Middle Persian Kaf Kof.