The flood (a.k.a. Noah's Flood) is the worldwide cataclysmic event that utterly devastated the earth sometime between 2,000 and 3,000 BC. According to Genesis, chapters 6 through 9, earth's humanity had degenerated into such a cesspool that God, in His sovereignty, decided to purge the earth. He graciously chose to spare a single human family and a variety of representatives from the animal kingdom to repopulate the earth after the flood waters receded. Noah, the patriarchal head of this chosen family, the man who "found grace in the eyes of the LORD" (Genesis 6:8), was commanded to build a massive ark. This ark (a.k.a. Noah's Ark) was the implement of their salvation. In the aftermath, eight human survivors disembarked from the ark. These eight were all that remained of the human race: Noah, his wife, their three sons, and their sons' three wives. The flood - Universal Tradition
According to the Bible, every human to have lived since the flood is a direct descendent of Noah's small remnant. It logically follows that despite the relative isolation of the various cultures that have thrived since the Flood, because every culture descended directly from the flood's survivors, traditions of this traumatic event ought to be abundant and universal, having been passed down generation to generation. Indeed, flood traditions are both abundant and universal. Literally hundreds of flood traditions have been preserved throughout the world, abounding in Europe, Asia, Africa, Australia, and the Americas. Collectively, these flood traditions serve to corroborate the Bible's Genesis flood account. These surviving flood traditions include two of the oldest known stories to have survived the ravishment of time's passage: China's "Hihking Classic" and Babylon's "Epic of Gilgamesh." These flood traditions are remarkably consistent, considering the relative isolation of the cultures, the length of time since the Flood, and the human tendency to embellish and exaggerate stories over time. The flood - Physical Evidences
The flood "myth" is not just some ancient allegory meant to teach us about God's judgment on sin. The flood was a real historical event and earth's crust bears witness to this in many compelling ways. Consider the fossil record: billions of dead things buried in sedimentation ("laid-down-by-water rock") found all over the earth. Geologist Dr. John Morris explains, "Sedimentary rocks, by definition, are laid down as sediments by moving fluids, are made up of pieces of rock or other material which existed somewhere else, and were eroded or dissolved and redeposited in their present location."  Over 70% of the earth's surface rock is sedimentary rock (the rest of earth's surface rock is volcanic igneous and metamorphic rock). In these sedimentary rock layers, geologists find some very odd features. For example, fossilized trees buried at all angles, upside-down and right-side-up, often passing through multiple rock layers, obviously the result of a marine cataclysm. These "polystrate" fossils (poly, meaning more than one; strate, meaning rock layer) are a worldwide phenomenon.
Consider the ratios of dead things we find buried in this sedimentary rock: "95% of all fossils are marine invertebrates, particularly shellfish. Of the remaining 5%, 95% are algae and plant fossils (4.74%). 95% of the remaining 0.25% consists of the other invertebrates, including insects (0.2375%). The remaining 0.0125% includes all vertebrates, mostly fish. 95% of the few land vertebrates consist of less than one bone. (For example, only about 1,200 dinosaur skeletons have been found.)" 
Also consider the abundant fossil remains of marine life found atop every mountain range in the world. For example, clusters of hundreds of gigantic (300kg/650lbs) oysters found atop the Andes Mountains in South America.