Tafsir of Al-Kahf 18:41

أو يصبح ماوها غورا

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:

قل أرءيتم إن أصبح ماوكم غورا فمن يٲتيكم بماء معين

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:

أو يصبح ماوها غورا فلن تستطيع له طلبا

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