Matthew Henry Commentary Genesis 7:17-20

"And the flood was forty days upon the earth; and the waters increased, and bare up the ark, and it was lifted up above the earth. And the waters prevailed, and 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 mountains that were under the whole heaven were covered. Fifteen cubits upward did the waters prevail; and the mountains were covered." — Genesis 7:17-20 (ASV)

The flood was increasing for forty days. The waters rose so high that the tops of the highest mountains were overflowed by more than twenty feet. There is no place on earth so high as to put people beyond the reach of God's judgments. God's hand will find out all his enemies (Psalms 21:8). When the flood increased in this way, Noah's ark was lifted up, and the waters that broke down everything else bore up the ark. That which signifies death to death for unbelievers, signifies life to life for the faithful.