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? v'Sera'eh ha'Yabashah" (Bereishis 1:9). Initially, the element of water was mixed with the element of air. Therefore, it covered the entire earth all around. The Raki'a was made via the light, i.e. the vapors began to rise. After all the water was vapors mixed in the air, they rose and became clouds and rain. This is the law of rain from vapors that rise now; the air mixed with them separates from them. Water (rain) without air falls. A Midrash compared this to 10 inflated pouches in a hall. One needed their place, so he removed their air. So Hashem removed the wind from the water; it gathered and constricted and descended to the Okainus (ocean), and the dry land was seen. This is how He founded the land so it will never falter.