Surah 18. Al-Kahf, Ayah 17

Lit., "while they were in a broad cleft thereof". The cave evidently opened to the north, so that the heat of the sun never disturbed them: and this, I believe is an echo of the many Qur'anic allusions to the happiness of the righteous in paradise, symbolized by its "everlasting shade" (see, in particular, surah {4}, note [74], on the metaphorical use of the term zill in the sense of "happiness").

 While they slept in the open space of the cave, fresh air blew through the cave, but they were protected from the heat of the sun.

In the latitude of Ephesus, 38 degrees north, i.e., well above the sun's northern declination, a cave opening to north, would never have the heat of the sun within it, as the sunny side would be the south. If the youths lay on their backs with their faces looking to the north, i.e., towards the entrance of the Cave, the sun would rise on their right side, declining to the south, and set on their left sides, leaving them cool and comfortable.

The youths, having faith and trust in Allah, found safety and refuge in the Cave. They were protected from the persecution and violence of the heathen. Their prayer (xviii. 16) was heard.