Why did he specify the north?
Ibn Ezra: It is where the world is settled.
Malbim: He clarifies his question, why Hashem does not punish Resha'im without opposing the Ma'arachah. The elements of water and Ru'ach are not connected to the Galgalim. They have their own natural laws, which changed after Beri'as ha'Olam. Initially, the land was covered with water all around; this is its nature. Hashem changed it, and gathered the water to one place. Initially air was mixed with water; Hashem separated it, and vapors rise from the water and return as rain. Early Chachamim said that Tzafon is the bottom of the world. It is still covered with water (the Okainus 1 ), like its creation; it is not settled.
Chazal call Okainus the ocean that surrounds the land (Asia, Africa and Europe). Nowadays, we see that the Pacific ocean covers about half the world. It is 'the bottom of the world', i.e. if we consider Eretz Yisrael to be on top. (PF)
How is Hashem "Toleh Eretz Al Belimah"?
Chulin 89a #1: The world exists in the merit of people like Moshe and Aharon [who considered themselves Bli Mah, i.e. nothing. They said "v'Nachnu Mah".]
Chulin 89a #2: The world exists in the merit of people who are Bolem (seal) the mouth at the time of an argument.
Rashi: The north and land have no foundation. The strength of Hashem's arms suspends them in the air.
Rashi citing Dunash: It is suspended on a flat plain. This is like "b'Meseg va'Resen Edyo Livlom."
Ibn Ezra: The land is in the middle of the Galgalim.
Malbim: The dry land is suspended on water. Belimah is the statute that is Bolem (closes) against the water and stops it from covering the land.
