Why is water HOT at place of setting of sun in the story of Dhulqarnain?
Verse 18:86:
حَتَّىٰٓ إِذَا بَلَغَ مَغْرِبَ ٱلشَّمْسِ وَجَدَهَا تَغْرُبُ فِى عَيْنٍ حَمِئَةٍ
When he (i.e. Dhulqarnain) reached the far west, he found the sun setting, he found it setting in a spring of HOT WATER/dark mud,
Ibn Kathir tells us the reason for the hot water there. It was hot, while the western edge of FLAT Earth is CLOSER to the spot where the sun sets.
Ibn Kathir wrote under the commentary of verse 18:86 (link):
وقال ابن جرير والصواب أنهما قراءتان مشهورتان، وأيهما قرأ القارىء فهو مصيب، قلت ولا منافاة بين معنييهما إذ قد تكون حارة لمجاورتها وهج الشمس عند غروبها، وملاقاتها الشعاع بلا حائل
Ibn Jarir said, “There are two well-known recitations (i.e. the sun disappears in black mud, or the sun disappears in the spring of hot water) and whichever one the reciter recites is correct.” And I (i.e. Ibn Kathir) say, there is no contradiction between their two meanings, as it may be the water is hot because it is near the glare of the sun at sunset (i.e. the western edge of the flat earth is close to sun), and it meets the rays without a barrier.
Similarly, Quran also gives the same impression that the far eastern edge of flat earth is also hot.
Quran 18:90:
حَتَّىٰٓ إِذَا بَلَغَ مَطْلِعَ ٱلشَّمْسِ وَجَدَهَا تَطْلُعُ عَلَىٰ قَوْمٍ لَّمْ نَجْعَل لَّهُم مِّن دُونِهَا سِتْرًا
Until, when he (i.e. Dhulqarnain) came to the rising place of the sun (i.e. the far eastern edge of Flat Earth), he found it rising on a people for whom We (Allah) had provided no shelter against the sun.
Ibn Kathir wrote under the commentary of this verse (link):
وقال قتادة ذكر لنا أنهم بأرض لا تنبت لهم شيئاً، فهم إذا طلعت الشمس، دخلوا في أسراب، حتى إذا زالت الشمس، خرجوا إلى حروثهم ومعايشهم ... فقال لهم أهلها لا تطلعن عليكم الشمس وأنتم بها، قالوا لا نبرح حتى تطلع الشمس، ما هذه العظام؟ قالوا هذه جيف جيش طلعت عليهم الشمس ههنا فماتوا، قال فذهبوا هاربين في الأرض.Qatadah said, "It was mentioned to us that they were in a land where nothing grew (due to heat), so when the sun rose they would go into tunnels until it had passed its zenith, then they would come out to go about their daily lives and earn themselves a living ... And once an army came to them, and they (the locals) said to them: “You should not be here when the sun rises.” But they (didn't listen to the locals and) said, “We will not leave until the sun rises. But what type of bones are these that are lying here?” They (the locals) said, "These are the carcasses of an army (which came here before you). The sun rose on them here, and they died." Upon hearing that, the army went back from there.
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