Often when this question comes up it is because many, perhaps most, scientists these days are trying to say that if there was a flood of cataclysmic proportions, it was only a localized, or regional flood. That is, it occurred only where there was life, in particular, human life.
The scientists and others can’t deny there was a great flood, because: 1) There is sea fossil life on even the summits of some of the world’s highest mountains; and 2) Cave drawings, and other recorded art and written works by primitive civilizations around the globe record in their legends a great flood of some sort.
God’s Word says:
“And the flood was forty days upon the earth; and the waters increased, and bare up the ark, and it was lift up above the earth. And the waters prevailed, and were increased greatly upon the earth; and the ark went upon the face of the waters. And the waters prevailed exceedingly upon the earth; and all the high hills, that were under the whole heaven, were covered. Fifteen cubits upward did the waters prevail; and the mountains were covered” (Gen. 7:17-20).
The Bible says further:
“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 which was upon the face of the ground, both man, and cattle, and the creeping things, and the fowl of the heaven; and they were destroyed from the earth: and Noah only remained alive, and they that were with him in the ark” (Gen. 7:21-23).
Peter the apostle forewarns that the skeptics would be talking about such things in the last days.
“Knowing this first, that there shall come in the last days scoffers, walking after their own lusts, And saying, Where is the promise of his coming? for since the fathers fell asleep, all things continue as they were from the beginning of the creation. For this they willingly are ignorant of, that by the word of God the heavens were of old, and the earth standing out of the water and in the water: Whereby the world that then was, being overflowed with water, perished” (2 Pet. 3: 3-6).
Believe the scientists and the other scoffers, or believe God, who cannot lie. We weren’t there, so we must believe one or the other. It’s a matter of faith, but the geological record is on God’s side. The whole earth was covered at least 15 cubits above the highest mountain peak.