"All the rivers run into the sea; yet the sea is not full; unto the place from whence the rivers come, thither they return again." (Ecclesiastes 1:7)
As the ancients observed the mighty Nile and Euphrates and other great rivers flowing into the ocean, they could not help but wonder why the sea level never rose. They knew that many of the waters in the rivers came from rainfall, especially during floods, but they had only quaint notions, at best, as to where the rains originated. Not until the days of modern science did men discover that rainfall actually comes from the oceans via evaporation and atmospheric transportation.
But the Bible writers somehow seemed to know about the true nature of the hydrologic cycle thousands of years in advance of modern science. The rivers come from the same place to which they return--that is, the sea.
But how do the waters of the sea ever rise into the sky? "He maketh small the drops of water: they pour down rain according to the vapour thereof: Which the clouds do drop and distill upon man abundantly" (Job 36:27-28). Water droplets are made very small by the process of evaporation so they can be carried aloft by the up-rushing air forces over warm waters; later they "distill upon man abundantly."
There are other references in Scripture to different phases of this great hydrologic cycle, but one of the most significant is Isaiah 55:10-11: "For as the rain cometh down, and the snow from heaven, and returneth not thither, but watereth the earth. . . . So shall my word be that goeth forth out of my mouth: it shall not return unto me void, but it shall accomplish that which I please." The waters return to the skies only after doing their good work on the lands. Just so, the life-giving word of God returns to Him, not void, but full of the spiritual fruit for which He sent it. HMM