The Köpet Dag, Kopet Dagh, or Koppeh Dagh, also known as the Turkmen-Khorasan Mountain Range, is a mountain range on the border between Turkmenistan and Iran that extends about 650 kilometres along the border southeast of the Caspian Sea, stretching northwest-southeast from near the Caspian Sea in the northwest to the Harirud River in the southeast. Wikipedia