Concise Bible Dictionary:
This judgment of God upon the earth, when the whole world had become corrupt before Him, has often been thought to be a subject full of difficulties, the principal of which it may be well to consider. First, as to its extent, was the flood universal? Language can scarcely be more explicit than is the scripture on this point. We read that "all the high hills, that were under the whole heaven, were covered. Fifteen cubits upward did the waters prevail; and the mountains were covered. And all flesh died that moved upon the earth, both of fowl, and of cattle, and of beast, and of every creeping thing that creepeth upon the earth, and every man: all in whose nostrils was the breath of life, of all that was in the dry land, died. And every living substance was destroyed.... and Noah only remained alive, and they that were with him in the ark" (Gen. 7:19-23). After the flood God said He would not any more smite “everything living,” as He had done (Gen. 8:21); "neither shall there any more be a flood to destroy the earth" (Gen. 9:11: Compare also 2 Pet. 2:5; 2 Pet. 3:6-7). Words cannot be plainer than the above to signify a universal deluge: the world that then was is distinguished from the earth that now is, and it is easy for faith to accept God's statement. It was a miracle, and it would require as great a miracle to cover all the high hills in one district only, without the water flowing to other parts, as to submerge the whole earth. The quantity of water required to cover the whole earth could easily be formed by God the Creator of all things, and be dispersed into its elements afterward.
It has often been contended that as man only was the guilty creature, the destruction of all mankind would have entirely met the case. It might have been thus if God had so pleased, but He has taken pains to tell us that all cattle, beasts, and creeping things were destroyed; and we must believe Him. Man was the head of creation, and all was involved in the consequences of his sin, and there must be a new start under the figure of the death and resurrection of Noah in the ark. God commenced a new economy as to the earth, in connection with the sweet savor of Noah's sacrifice. The flood was about 1700 years after the creation of Adam, and it is impossible to say how many millions of people there were on the earth at the time, or how far they had been dispersed.
Another difficulty felt is as to the great number of species being all preserved in the ark, such, it is said, as 1500 mammalia, 6000 species of birds, and some hundreds of thousands of reptiles and insects! It is very probable that at that time a great many of these did not exist. God foreknew that the flood would sweep away the great bulk of them, and He could have restrained the forming of species, and have kept them to a comparatively few genera. Compare the statement that “every living creature” was brought to Adam to be named. All the original generic types then existing were gathered into the ark, from which the species, under many varying circumstances, may have greatly increased. This would be from natural causes, as has been known to have been the case, without in anyway agreeing with or falling under the modern theory of evolution. The clean animals were doubtless only four in number: the ox, the sheep, the goat, and the pigeon—those offered in sacrifice; the distinction between clean and unclean animals for food was made long after.
Again it has been asked, How could the animals have been fed for a full year? and what could have prevented the wild animals devouring one another? Scripture does not say how the animals were fed. God may have caused many of them to have slept the greater part of the time, as some do now constantly in the winter. In Paradise the green herb was the food for every beast, every fowl, and every creeping thing, as well as for man (Gen. 1:29-30); and they may not have become carnivorous until after the flood, when flesh was given to man to eat (Gen. 9:3). If, on the other hand, because sin had come in, they had been previously living on one another, God could have altered this while in the ark, as He certainly will do in the millennium (Isa. 11:6-9; Isa. 65:25; Ezek. 34:25). Men, and even professing Christians, scoff at this, because of their knowledge of physiology; but even history proves that carnivorous animals will feed upon vegetation when they cannot get animal food, and vice versa.
By faith Noah prepared the ark (Heb. 11:7). Everything concerning the flood was arranged by God; Noah had simply to follow out the instructions given. The same faith believes that it was fully carried out as described; and there is no real difficulty in the matter, except by shutting out God, which must not be, for it was His flood. The old world was then destroyed except those in the ark, and they were perfectly safe, for God shut them in. The promise was afterward given that God would not again destroy the world with a flood; but it is, alas, reserved to be destroyed by fire (2 Pet. 3:7, 10). This is a prophecy as little believed by many, as was the deluge that was proclaimed by Noah; but which will as certainly come to pass. The details of the deluge are given in full in Genesis 6-8. In almost all heathen countries there exist ancient traditions of the flood, though with many variations. The descendants of Noah would carry the record of the solemn judgment wherever they roamed. See ARK.