Surah Al Kahf Tafseer

أَوْ يُصْبِحَ مَاوُهَا غَوْرًا

Or the water thereof becomes Ghawran,

means, it disappears into the earth, which is the opposite of flowing water that seeks the surface of the earth.

So Gha'ir is to go lower. as Allah says:

قُلْ أَرَءَيْتُمْ إِنْ أَصْبَحَ مَأوُكُمْ غَوْراً فَمَن يَأْتِيكُمْ بِمَأءٍ مَّعِينٍ

Say:"Tell me! If your water were Ghawran, who then can supply you with flowing water!" (67:30)

meaning, water that flows in all directions.

And here Allah says:

أَوْ يُصْبِحَ مَاوُهَا غَوْرًا فَلَن تَسْتَطِيعَ لَهُ طَلَبًا

Or the water thereof (of the gardens) becomes deep-sunken (underground) so that you will never be able to seek it.

Ghawr is from the same root as Gha'ir and has a similar meaning, but is more intensive