The water prevailed more and more upon the earth, so that all the high mountains everywhere under the heavens were covered. The water prevailed fifteen cubits higher, and the mountains were covered. (Genesis 7:19-20)It had been forty days since the flood began. Peering out the ark's window at this time would have revealed a simple scene. A scene of only water— water, water, and more water. After forty days, nothing else was visible. Absolutely nothing.
In past days, perhaps Noah could see the tips of the highest mountains, just barely penetrating the water's surface. But as the hours and days passed, the highest mountains were left without air. One by one, they sunk into the watery abyss that the rest of the world had already perished in... but even then, the water still did not stop rising.
Instead, the water rose above the highest mountains by another 15 cubits— about 22.5 feet (270 inches) assuming an 18 inch cubit (for more information on the length of a cubit see How Big Was Noah's Ark?).
And just to remove any doubt about this historical event— just to make sure that no one would misunderstand the worldwide impact of the flood, Scripture states that all the high mountains... were covered. This statement alone would have been enough to make it clear that every mountain throughout the entire earth was covered.
Scripture, however, provides even more detail. Not only does Scripture say that all the high mountains were covered, but Scripture says that all the high mountains everywhere under the heavens were covered.
Evidence for a Global Flood: Water Above The Mountains (Part 2) - Genesis 7:19-20
The Destruction of The City of Enoch (Part 1) - Genesis 7:17-18
The Pre-Flood World Soon To Be Blotted Out - Genesis 6:7
State Of The World (2469 BC): 120 Years Before The Flood - Genesis 6:3
State of the World (2349 BC): The Flood - Genesis 7:11