What is the meaning of the verse that says "a murky spring" in reference to Dhul-Qarnayn?

Ibn Kathir says in his interpretation of the verse: ﴿Until, when he reached the setting place of the sun, he found it [as if] setting in a spring of dark mud﴾ [Al-Kahf:86], that Dhul-Qarnayn reached the farthest point of the earth from the west, and saw the sun setting in the surrounding ocean. This is a sight that everyone who reaches the coast sees, even though the sun does not depart from its orbit. The word "hami'ah" (حمئة) was said to mean "hot," and this is the reading of Ibn Kathir, Nafi', Abu Amr, and Hafs from Asim. Whereas the others read "hamiyah" (حامية), meaning having a حمأة (black mud). It is possible to combine the two readings by saying that it was stones and had mud.