In the last paragraph of Genesis seven, the flood arrives. Now, we don’t know whether the people of Noah’s time had ever experienced flooding before. The world was smooth-surfaced compared to today.
But, this was different even if they knew what a flood was. The Hebrew word is mabbuwl. It only ever refers to the flood of Noah, not ordinary flooding. The word mayim is used for normal flooding 1.
No other flood would ever match this one. God says later “I establish My covenant with you; and all flesh shall never again be cut off by the water of the flood, neither shall there again be a flood to destroy the earth.” (Genesis 9:11) On that basis alone, we know for certain that it was global in scope, because otherwise God has violated His covenant with us (to never send a flood like Noah’s again.) We know God doesn’t lie or make mistakes, so the flood of Noah was far greater than any other flood in history.
The other reason we know for certain that it was global is because it would be absurd to have to build the biggest boat ever built. If the flood were localized, Noah could have walked out in 120 years. All he had to do to get to the opposite side of the earth would be to walk a mile every three days.
But, in case some would say there’s still wiggle-room for a local flood, there’s the description itself. “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)
But, I still hear some question this: “But, there’s not enough water on the whole planet to cover Mt. Everest!” Remember folks, Mt. Everest was created AFTER the flood, not before it. I live in Minnesota, where the highest point in the state is Eagle Mountain. As anybody who has visited both Eagle Mountain can tell you, Eagle Mountain is no mountain. It’s more of a bump on the earth. It really doesn’t deserve its name.
Before the flood, the earth was relatively flat. Or, at least a lot flatter than it is now. A so-called mountain to them would be a hill now. And, the earth has plenty of water to cover anything lower than two miles in height.
Finally, this flood killed every land animal and even every bird, that was not on the ark. Birds can survive quite a while migrating, but the longest ever measured is the bar-tailed godwit, that flew over 7,000 miles non-stop 2https://www.nationalgeographic.com/animals/2007/09/alaska-bird-longest-mirgation/3. But, they take only a month to do it, and their reserves are spent. To survive a whole year in flight isn’t possible. So, yes, even the birds all died.