101. Remnants of such an ancient wall are found in the Dāriyā pass of the Caucasus mountain range. Explorers mention it in their travelogues. This wall was 50 miles long, 290 feet high and 10 feet wide. Imām Amīn Aḥsan Iṣlāḥī writes:
… It is mentioned in historical narratives that the Abbasid caliph Wāthiq deputed a team of 50 people to research about this wall. This team was able to locate it. People ascribe this wall to Dāriyā Naushayrwān but evidence is more in favour of Cyrus building it. For example, this much has been verified that the northern border of Cyrus’ empire extended to the Caucasian mountains. Conquering such a vast area could only have been possible when it had been militarily attacked. Even today a city and a river by the name of Koresh exists in the Caucasus mountains. The iron wall is called Gorgan which is an altered version of Koresh. This wall has been built with metal between two mountains and its lower part has an outlet for rain water. (Amīn Aḥsan Iṣlāḥī, Tadabbur-i Qur’ān, vol. 4, 621)
Al-Bayan, Surah Kahf, footnote 101.