According to the description of the Creation in the book of Genesis “God also said: Let the waters that are under the heaven be gathered together into one place; and let the dry land appear. And it was so done.” The difficulty in explaining this text was to account for what became of these waters. Great as are the seas, they were not considered large enough to absorb all the primordial waters, and consequently arguments were elaborated in favor of the existence of vast reservoirs within the earth. Bede, for example, was of the opinion that the waters under the firmament at first took the form of clouds and that when they became condensed and fell as rain the water was sequestered in caverns of the earth’s interior.[259] Of even greater significance was the assertion that God had gathered the waters below the firmament into “one” place. This could mean nothing else than that all the waters of the earth, whether in subterranean reservoirs, oceans, lakes, rivers, or in the atmosphere, must be connected and must constitute a unit. Probably with this idea in mind Isidore wrote: “The abyss is the deep water which cannot be penetrated, whether caverns of unknown waters from which springs and rivers flow, or the waters which pass secretly beneath, whence it is called abyss. _For all waters or torrents return by secret channels to the abyss which is their source_” (Brehaut’s translation).[260] Certainly most medieval theorizing about the origin of springs and rivers[261] was dependent on the doctrine of a “congregation of waters.” In further elaboration of this doctrine it was often said that the water of the sea found its way by underground channels to the Garden of Eden and returned again to the sea, flowing first through a subterranean passage and thence through the four rivers of Paradise. Augustine maintained that the words of Genesis (ii, 6), “But a spring rose out of the earth, watering all the surface of the earth,” mean that all the waters of the earth come from a single source.[262] Rainfall as a source of springs and well water, however, was also recognized,[263] and Gregory of Nyssa accepted and elaborated the classical theory of the transmutation of earth into water.[264] THE NILE FLOOD The strange phenomenon of the flood of the Nile brought forth no new theories during the Middle Ages, and Isidore, whose words were most often copied, reverted to the explanation of Thales that the flood was caused by the building of sand bars at the mouth of the river during the summer when the etesian winds blow.[265] THE EARTH UPON THE WATERS