Answer: When one examines the biblical passages, it is clear that the flood was global. Genesis 7:11 states that “all of the fountains of the great deep were broken up, and the windows of heaven were opened.” It is apparent from Genesis 1:6-7 and 2:6 that the pre-flood environment was much different from that which we experience today. Based on these and other biblical descriptions, as well as the fossil record and present geological findings, it is reasonably speculated that at one time the earth was covered by some kind of water canopy. This canopy could have been a vapor canopy or could have consisted of rings, somewhat like Saturn’s ice rings. This, in combination with a major layer of water underground, both being released upon the land (Genesis 2:6) would have resulted in a global flood.
The clearest verses that show the extent of the flood are Genesis 7:19-23: “And the waters prevailed exceedingly on the earth, and all the high hills under the whole heaven were covered. The waters prevailed fifteen cubits upward, and the mountains were covered. And all flesh died that moved on the earth: birds and cattle and beasts and every creeping thing that creeps on the earth, and every man. All in whose nostrils was the breath of the spirit of life, all that was on the dry land, died. So He destroyed all living things which were on the face of the ground: both man and cattle, creeping thing and bird of the air. They were destroyed from the earth. Only Noah and those who were with him in the ark remained alive.”
In the above passage we not only find the word “all” being used repeatedly, but we also find phrases such as “and all the high hills under the whole heaven were covered,” “the waters prevailed fifteen cubits upward, and the mountains were covered” (enough to allow the ark to pass over them safely), and “all flesh died that moved on the earth: birds and cattle and beasts and every creeping thing that creeps on the earth, and every man,” etc. If these descriptions are not meant to describe a universal flood covering the whole earth, I don’t know how God could have made it clearer. Also, if the flood was only localized, why did God instruct Noah to build an ark instead of merely causing the animals to migrate and telling Noah to do the same? And why did He instruct Noah to build an ark large enough to house all of the different kinds of land animals found on the earth today. One might note that even dinosaurs start out small, and it would not have been necessary for Noah to have brought fully grown animals onto the ark.
God did instruct Noah to put two of every land animal (aquatic wildlife was excluded) onto the ark (Genesis 6:19-22) with the exception of ceremonial clean animals and for all birds, of which he was to have seven of each kind on the ark (Genesis 7:2-3).
Peter also describes the universality of the flood in 2 Peter 3:6-7 in which he states: “by which the world that then existed perished, being flooded with water. But the heavens and the earth which are now preserved by the same word, are reserved for fire until the day of judgment and perdition of ungodly men.” In these verses Peter compares the “universal” coming judgment to the flood of Noah’s time and states that the world that then existed was flooded with water. Also, God’s promise (Genesis 8:21; 9:11, 15) never again to send such a flood has been broken repeatedly if it were only a local flood. Further, all men in the world today are said to have descended from Noah’s three sons (Genesis 9:1, 19) and many later Biblical writers accepted the historicity of the worldwide Flood (Isaiah 54:9; 1 Peter 3:20; 2 Peter 2:5; Hebrews 11:7). Lastly, the Lord Jesus Christ believed in the universal Flood and took it as the type of the coming destruction of the world when He returns (Matthew 24:37-39; Luke 17:26, 27).
There are many extra-biblical evidences that point to a worldwide catastrophe such as a global flood. The vast fossil graveyards found on every continent, the large amount of coal deposits that would require the rapid covering of vast quantities of vegetation, the fact that oceanic fossils are found upon mountain tops around the world, the over 270 flood stories from all parts of the world, and the large extent of geological formations showing vast layers of sedimentary deposits (including those found in the Grand Canyon) all lend credence to the occurrence of a global flood.