Surah Al-Kahf 18:86

حَتَّىٰٓ إِذَا بَلَغَ مَغْرِبَ ٱلشَّمْسِ وَجَدَهَا تَغْرُبُ فِى عَيْنٍ حَمِئَةٍ وَوَجَدَ عِندَهَا قَوْمًا ۗ قُلْنَا يَـٰذَا ٱلْقَرْنَيْنِ إِمَّآ أَن تُعَذِّبَ وَإِمَّآ أَن تَتَّخِذَ فِيهِمْ حُسْنًا

Until, when he reached the setting of the sun [i.e., the west], he found it [as if] setting in a body of dark water, and he found near it a people. We [i.e., Allāh] said, "O Dhul-Qarnayn, either you punish [them] or else adopt among them [a way of] goodness."

Translation: Saheeh International

Tafsir

until when he reached the setting of the sun the place where it sets he found it setting in a muddy spring ‘ayn hami’a a spring containing ham’a which is black clay its setting in a spring is described as seen from the perspective of the eye for otherwise it is far larger in size than this world; …

Excerpt from Tafsir al-Jalalayn, commentary on 18:86, via Quran.com. The reader has the full commentary and 9 more tafsirs.