First, the biblical passages regarding the flood make it clear that it was global. “In the six hundredth year of Noah's life, in the second month, the seventeenth day of the month, the same day were all the fountains of the great deep broken up, and the windows of heaven were opened.” Genesis 7:11
Genesis 1:6-7 and 2:6 state that the pre-flood environment was much different from that which we experience today. Based on these and other biblical descriptions, 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 it might have consisted of rings, somewhat like Saturn's ice rings. This, in combination with a layer of water underground, 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. Regarding 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. 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.”
In the above passage, we not only find the word “all” being used repeatedly, but we also find all the high mountains under the entire heavens were covered, the waters rose and covered the mountains to a depth of more than 15 cubits or twenty feet, and every living thing that moved on the earth perished. These descriptions clearly describe a universal flood covering the whole earth. Also, if the flood was localized, why did God instruct Noah to build an ark instead of merely telling Noah to move and causing the animals to migrate? 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? If the flood was not global, there would have been no need for an ark.
Peter also describes the universality of the flood in 2 Peter 3:6-7, where he states, “Whereby the world that then was, being overflowed with water, perished: But the heavens and the earth, which are now, by the same word are kept in store, reserved unto fire against 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 existed then was flooded with water. Further, many biblical writers accept the historicity of the worldwide flood (Isaiah 54:9; 1 Peter 3:20; 2 Peter 2:5; Hebrews 11:7). Lastly, Jesus the 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).
By Day 150 of the Flood catastrophe the floodwaters had risen until they covered “all the high mountains under the whole heaven” (Genesis 7:19). After that “the waters receded from the earth continually” (Genesis 8:3), a process that took about seven months.
As the water receded from the continents it flowed into the oceans. It only takes a quick look at a globe of the earth to appreciate that the water indeed sits in the oceans. The Pacific Ocean alone takes up almost half the earth’s surface.
The top part of the earth, called the crust, sits on top of the mantle (about 3,000 km (1,900 miles) thick), which in turn sits on the earth’s iron core.
The continental crust is about 40 km (25 miles) thick, while the thickness of the oceanic crust is only around 7 km (5 miles). Up-and-down movement of the crust during Noah’s Flood, called differential vertical tectonics, explains how the waters drained from the continents. On a smaller scale, mountain ranges would have risen and valleys sunk.
There are many extra-biblical evidences that point to a worldwide catastrophe such as a global flood. There are vast fossil graveyards found on every continent and large amounts of coal deposits that would require the rapid covering of vast quantities of vegetation.
Oceanic fossils are found upon mountain tops around the world. Cultures in all parts of the world have some form of flood legend. All of these facts and many others are evidence of a global flood.