God’s second day of creation involved the "division of the waters," that is, He created the firmament (rakiya) to separate the watery sphere of the earth from its atmosphere
(2 Pet 3:5). The waters under the firmament initially constituted the earth (vv. 9-10) and the waters above constituted the edge of the outer limits of the firmament (cf. Ps. 148:4). This firmament, also named shamayim (heaven) and perhaps etymologically related to the word for mayim (water), came into existence the second day.
Likewise, the Psalmist recounts the “stretching out of the earth” upon the waters: