Radak: They are what is below the ground. They are b'Yado, just like "Gavhei Shamayim" (Iyov 11:8) are.

Malbim: The nations understand Hashem from His general deeds. He created land, mountains and seas. They see that He established existence with wondrous Chachmah, and for a purpose. Via this, they recognize also that He oversees endurance of the creation via natural laws that He made at creation. They think that He prepared the land according to His will alone; He has no connection to people's deeds based on choice. "Mechkerei Aretz" are b'Yado, and not b'Yad people; they do not depend on people's actions.

Rashi: It is an expression of height, like a bird that flies [high above the ground].

Radak: The verse mentions the depths below the ground, and the high mountains above the ground. "So'afos" is like "v'So'afos Re'em Lo" (Bamidbar 23:22).

Malbim: Even though mountains were 'born' after the creation, this is only due to Hashem, and the natural laws that He established.