The Genesis Flood was not merely regional; it covered the entire world.
Dr. James J. S. Johnson
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. (Genesis 7:19-20)
The apostle Peter predicted (in 2nd Peter 2:5 & 3:6) that some people would intentionally ignore the historic fact of the worldwide Flood, because that Flood was a haunting reminder that God punishes sin, especially when His forgiveness is rejected. Before that the Lord Jesus Himself clearly taught that the Genesis Flood destroyed all of the earth, with Noah’s family being the only humans to survive (Matthew 24:39). Jesus also said that Genesis was authoritative (John 5:44-49), and the Genesis record itself (e.g., in Genesis 7:19-20) chronicles how the floodwaters rose and eventually covered the entire earth, so that even the highest mountains were covered by at least 15 cubits (which is about 24 feet). Also, using the rainbow as a covenant, God promised to never again send a flood like the one Noah and family survived (Genesis 9:11), yet many regional floods have come and gone since then.
There are 2 other major evidences that the Genesis Flood was global, not just regional: (1) Flood mega-sequences in sedimentary rock layers, documented by Dr. Tim Clarey, cover multiple continents, not just regions of the world; (2) historical memories of the Flood, such as global flood legends and Chinese pictographs, only makes sense if the Flood was worldwide, since Flood memories are transmitted in the lore of many different peoples (e.g., Chinese, Polynesian islands, Alaskan natives, Mayas, Africans, Greeks, etc.) who are separated by language and geography.
It was more than catastrophic; it was a global cataclysm. For more analysis of this important fact of Earth’s well-documented Flood history, consider these short articles: