Radak: He made the nature of the earth to descend. The low place is the middle of the Galgal. It is on its base, so the world will never falter. If it would slip to one of the six sides (the four directions, up or down), it would ascend; Hashem made that it descends and does not ascend.

Malbim: This refers to "Yikavu ha'Mayim