The Bible states that in the flood of Noah’s Day all of humanity was destroyed except for the eight people who were on the ark (2 Peter 2:5, Matthew 24:39).
Genesis 6 describes how mankind had chosen the way represented by the tree of the knowledge of good and evil, and their godless ways were dominating society. Something had to be done and God decided to destroy them and start all over again with just eight people.
He instructed Noah to build an immense ark (Genesis 6:14). If we accept a cubit as being about 18 inches, the ark would have been 450 feet long, around 75 feet wide and 45 feet high. These proportions would have made it very stable and more like a barge than a boat.
Noah was told to take all the beasts of the field into the ark, made up of one pair of unclean animals and seven pairs of clean animals (Genesis 7:2), proving that the laws of clean and unclean foods were well known to mankind long before Moses.
Some question if the flood was actually worldwide. We are told it covered "all the high hills under the whole heaven...," meaning all the earth was covered by water. The waters prevailed above the highest mountains by 15 cubits (about 22 feet) and scripture is very clear that all mankind and animals that breathed air (excluding fish of course) died (Genesis 7:19-21) The Revell Bible Dictionary (article "Ararat," page 83) supports this view.
Also Genesis 9:12-15 tells us God set a rainbow in the clouds to be a sign He would never again destroy the earth by a flood. If the Flood were local, then God did not fulfill His word. There have been many local floods, which have killed many people since the Genesis Flood.
We find stories of the Flood all over the world. The International Standard Bible Encyclopedia (article "The Deluge of Noah") gives details of the Babylonian flood story and there are ‘flood’ stories in most of the ancient civilizations. Mayans, Indians, Greeks, Chinese and many others have a story of the Flood, and most of them have the common theme that God (or gods) got angry with mankind and brought about the devastation.
We can learn from the example of Noah’s faith today. He believed God and began to build the ark and probably preached about the need to repent and what was prophesied to come for 120 years (Genesis 6:3).
2 Peter 3:3-6 tells us there will be "scoffers" just before Christ returns, believing their own theories before the Bible (Romans 1:20-22). Verse 11 states the earth will be destroyed by fire, if mankind does not repent, just as it was once destroyed by a flood. The ancient Flood is a warning for us, and we are exhorted to pay heed to how we conduct ourselves.