Tafsir of Al-Kahf 18:61

فلما بلغا مجمع بينهما نسيا حوتهما

But when they reached the junction of the two seas, they forgot their fish, He had been commanded to carry a salted fish with him, and it had been said to him, when you lose the fish, that will be a sign that you have reached the right place. So they set out and traveled until they reached the junction of the two seas, where there was a spring called `Ayn Al-Hayat (the Spring of Life). They went to sleep there, and the fish felt the drops of that water, so it came back to life. It was in a vessel with Yusha`, upon him be peace, and it jumped out of the vessel towards the sea. Yusha` woke up and the fish fell into the water and started to swim through the water, leaving a track or channel behind it. Allah said:

فاتخذ سبيله في البحر سربا

and it took its way through the sea as in a tunnel. meaning, like going through a tunnel on land. Ibn Jurayj said, "Ibn Abbas said, `It left a trace as if it were a rock.'"