Surah 18 (al-Kahf) recounts the legend of Dhul-Qarnayn ('the two-horned one'), a righteous king who travels to the ends of the earth. The narrative parallels (and almost certainly adapts) the Syriac Christian Alexander Romance (the Neṣḥānā d-Aleksandrōs), a 6th-century legendary biography of Alexander the Great, which was widely circulated in the Near East before Muhammad's time. Kevin van Bladel, in 'The Alexander Legend in the Qur'an 18:83–102' (in Gabriel Said Reynolds, ed., The Qur'an in Its Historical Context, Routledge, 2008, pp. 175–202), demonstrates that every stage of the Quranic passage has a close parallel in the Syriac legend and often in the same order: the righteous-king traveler, the journey to the sun's setting, the journey to the sun's rising, the encounter with a barbarous people, and the construction of a barrier against Gog and Magog. Most decisively, the water at the place where the sun sets is described as 'fetid/black' in both texts — the Syriac saryā and the Arabic ḥamiʾa are cognate synonyms, a near-impossible coincidence if the Quran were independently composed. The Syriac legend describes Alexander sailing until he reaches Oceanus, the fetid sea which surrounds the earth, and seeing the sun sink 'in a pool of mud and fetid slime.' Tommaso Tesei, in The Syriac Legend of Alexander's Gate: Apocalypticism at the Crossroads of Byzantium and Iran (Oxford University Press, 2023), independently corroborates van Bladel's thesis, arguing that an earlier stratum of the Syriac text was composed under Justinian I (r. 527–565) and that the Quranic account shares multiple streams of narrative elements from this tradition in the same sequence — further tightening the case for direct literary dependency.

Q 18:86 says Dhul-Qarnayn 'reached the setting of the sun' (maghriba l-shams) and 'found it setting in a spring of dark/muddy water' (taghrubu fī ʿaynin ḥamiʾah). The Arabic ʿayn is unambiguously a water-spring; ḥamiʾah means 'muddy' or 'black' (a textual variant reading is ḥāmiya — 'hot'). Both readings describe a physical location on earth where the sun physically sets into a body of water.

Classical tafsir treated the description as substantially geographical. Al-Tabari, in his Jāmiʿ al-Bayān ʿan Taʾwīl Āy al-Qurʾān, preserves multiple transmitted reports identifying the location as a real destination on the western edge of the inhabited world; while he allows that the scene reflects what Dhul-Qarnayn observed, he still transmits these geographical reports without dismissing the physical imagery. Ibn Kathir, in his Tafsīr al-Qurʾān al-ʿAẓīm, similarly transmits these geographical reports — noting the spring was 'black, muddy and stinking' — and connects the verse directly to the hadith material about the sun prostrating beneath Allah's throne (Bukhari 3199), treating the setting-in-the-spring and the prostration as two aspects of the same cosmological framework. Al-Qurtubi, in his al-Jāmiʿ li-Aḥkām al-Qurʾān, is notable for being among the earliest classical commentators to introduce a phenomenological hedge — he proposes that the sun set 'as if' in a spring from Dhul-Qarnayn's visual perspective, analogising it to a sailor who sees the sun appear to set in the sea — but this is precisely the apologetic move the verse's grammar does not support (see Common Muslim Response 1 below), and it shows that even classical mufassirs felt the cosmological pressure of the literal reading. The verse itself uses the indicative 'found it setting,' not 'appeared to him.' The cosmological reading is further anchored by Abu Dawud 4002, in which Muhammad himself tells Abu Dharr that the sun 'sets in a spring of warm water' — the Prophet citing Q 18:86 as literal geography. Q 18:90, parallel to 18:86, says Dhul-Qarnayn 'reached the rising of the sun' and 'found it rising on a people' — the same physicalist framework.

The astronomical fact: the sun does not set into anything. It is a star approximately 150 million kilometres from Earth, around which Earth orbits. Its 'setting' is a perspectival illusion produced by Earth's rotation. There is no spring, lake, or sea on Earth into which the sun descends. The verse describes a flat-earth cosmology in which the sun has a fixed daily destination on the planetary surface — a worldview common in pre-modern Near Eastern myth (Egyptian Ra in his solar barque, Greek Helios in his chariot) and incompatible with basic astronomy.

This verse cannot be defended as poetic without conceding that other verses (e.g. Q 36:38, 'the sun runs to a resting place for it') and the parallel Hadith literature (Bukhari 3199, where the sun prostrates beneath Allah's throne) construct a coherent flat-earth solar cosmology. The 'poetic' defence requires that we know in advance which verses are literal and which are figurative — a knowledge available only after modern astronomy.