His traveling and reaching the Place where the Sun sets (the West)

فَأَتْبَعَ سَبَباً

(So he followed a way.) Ibn `Abbas said that he followed different routes to achieve what he wanted.

The statement in verse 86: حَتَّىٰ إِذَا بَلَغَ مَغْرِ‌بَ الشَّمْسِ (until when he reached where the sun sets) means that he reached the far limit towards the West beyond which there was no populated area.

The word: حَمِئَةٍ (Hami'ah) in the succeeding phrase: فِي عَيْنٍ حَمِئَةٍ (into a miry spring) literally means dark marsh or mud carrying the sense of water beneath which there is dark mud and which causes the water it-self to appear black. As for the sense of his seeing the Sun setting into such a spring, it means that an onlooker perceived it as setting into the spring because there was no habitation or dry land in sight. This is like being in an open field while the Sun is setting where as far as one can see there appears to be no mountain, tree, or structure, naturally one who looks at the sight would feel that the Sun was sinking into the land mass.