Concise Quran Commentary
So Gog and Magog were unable to climb over it due to its height nor were they able to make a hole in it from beneath due to its hardness.
Tafsir for this Ayah
Surah 18 : Ayah 97
Surah 18 : Ayah 97
فَمَا ٱسْطَـٰعُوٓا۟ أَن يَظْهَرُوهُ وَمَا ٱسْتَطَـٰعُوا۟ لَهُۥ نَقْبًا
Thus they could not climb over it nor could they pierce it[48].