Creationists say this is Noah's flood. Scholars think it's the other way round - an existing myth was co-opted by the Israelites as part of their story, in much the same way as Christianity has co-opted pagan rituals such as Easter and Christmas.
Creationists also point to the fact that just about every culture that lives on a river plain has a flood story (but ignore the fact that cultures living in the mountains, don't)
Personally, I lean towards the Monty Python explanation. Picture a small family group stuck up a tree with flood waters rushing past. Teenage boy goes "Boy this is the biggest flood". Grandpa goes "Ha! You young whippersnappers wouldn't know what a big flood was! Why back in my grandfather's day there was a flood so big it covered the whole world." "Oh yeah? Then how come he didn't drown?" "Um er well ... he had a boat." "Well how come all the animals didn't drown then?" "Um er well .... it was a bloody big boat." "Well how did he have a boat" "And how did he get all the animals on board?" "Um er well ... God told him there was going to be a flood, so he built it......"
2. Logistics
The literalists have to pedal really hard to come up with rationalizations to explain how Noah's story could be true. The hilarious part about these excuses is that they all require miracles. None of them are miracles that are recorded in the bible. They are all miracles-I-need-to-make-my-story-work. (But if God was going to wave his magic miracle wand, why didn't he just kill off all the bad guys with a miracle ....?)
These made up miracles are a common feature of Creationist logic. The process works like this: my story is full of holes. I need a miracle to make it possible. God can do miracles. I really think my story is right. So God must have done a miracle. Case closed.
When the bible written down (about 500BC), they knew of only a few species of animals. They didn't know about other continents, they didn't know about the microscopic world. So may have seemed possible to get all the world's organisms on board a big boat, as long as you didn't think too hard about how to feed them for a year.
It was a very clever boat for a stone age family to build.
(This diagram comes from Answers in Genesis, an outfit committed to idea that Genesis is literally true - they see no problem with the idea that stone age technology could build a boat twice as big as any other wooden boat that man has ever been able to build, even in modern ship yards. It only needed a minor miracle to somehow be able to haul in hundreds of thousands of trees, saw it up without iron tools and build this huge boat, all the time carrying on subsistence farming.)
The diagram is also instructive. The Wyoming was the biggest wooden boat ever built, and its size caused an interesting problem. Because of the extreme length of the Wyoming and its wood construction, it tended to flex in heavy seas, which would cause the long planks to twist and buckle, thereby allowing sea water to intrude into the hold (see hogging and sagging). The Wyoming had to use pumps to keep its hold relatively free of water. In March 1924, it foundered in heavy seas and sank with the loss of all hands. The Wyoming was built with modern machinery, so they could cut all the boards to fit, and they could use modern steel fasteners, braces and joiners, as well as power pumps to get rid of the water that came in. Noah was working nearly 2000 years before the Iron Age, so even if he somehow managed the improbable feat of building a wooden boat one-and-a-half times bigger than the Wyoming, it would have sunk as soon as it hit the waves.
There've been attempts to build replica arks - A Dutch guy built one and had to equip it with steel pontoons just to make it safe enough to get across the English Channel.
Today we know that you would need to get tens of thousands of animals on board, plus their food. Answers in Genesis (AiG) excuse is that Noah only took on the 'kinds' of animals, such as the ancestors of cats. Which implies that there must have been extremely rapid evolution in the centuries after the flood - another miracle required, because even the keenest Evolution scientist doesn't think species form that fast. And of course need more miracles to figure out how to keep the carnivores from eating the herbivores ("God made them hibernate") how a few men could feed and clear manure for all those animals. ("They had advanced technology of which the knowledge was lost")
Literalists make it even harder for themselves because they want to believe that all the animals like dinosaurs made it onto the ark, and only became extinct afterwards. Another miracle needed to make all these dinosaurs go extinct without anyone noticing.
How did they get there? How did Noah collect kiwis and moas and tuataras and kangaroos and wallabies etc etc etc? (You would have thought Noah would have been a bit surprised at all the strange animals that appeared, but again, another of God's miracles; not newsworthy).
AiG's hypothesis is that there was only one continent in those days.
Problem with this idea is, how you get from one continent pre-flood, to the present-day situation? Bible literalists tell you that bible says there was turmoil before the flood waters receded, and that this was God sundering the lands.
The big problem is timing - if all this sundering happened during the flood (as their reading of the bible says), how did the kiwis and kangaroos etc get back to their lands (which must have been the right ones because that's where that trickster god put those fossils to test peoples' faith)? If the sundering happened after the ark had disgorged its passengers, the rapid movement of continents would have set off devastating tidal waves and earthquakes.
Why?
The biblical flood story portrays YHWH as an evil fuck-up.
(a) Why bother with a flood when he could just wish the evildoers out of existence ? Why drown all the world's babies and kittens?
(b) Why didn't this solution to the problem of evil actually work ???
4. Geology
Science has mounted 3 major attacks on the belief that the bible is literally correct.
The first was Galileo in the early 1600's. He showed that the Earth was't the centre of the Universe, as many religious writers thought.
The second was the new science of Geology in the early 1800's.
And the third, of course, was Darwin and Evolution in 1859.
Geology became important around 1800 because man started digging. Dug down for minerals, and dug across the land first for canals, then for railways. And as the canals and railways were cut, it became obvious that the land was made in layers or strata. And in many places, it was apparent how those strata were laid down. You could see rocks that were the same as stuff coming out of volcanos (basalt), rocks that were made of millions of sea shells and must have once been under the sea (chalk), rocks that contained fossilized trees and leaves and must have been laid down in the same way as swamps were today (coal), and rocks that had once been subject to intense heat and pressure (marble).
At first the scientists thought that these strata must have been laid down by Noah's flood, but it quickly became obvious that this couldn't be right. The little problem was that it was hard to see how all these layers could have formed in the 6000 years of biblical creation. But the big problem was that there were many different areas where there were obviously multiple events. There seams of coal covered by layers of chalk, then basalt from a volcano which no longer seemed to exist, and then more coal. There were areas where some strata were bent and twisted, but were then covered by undisturbed layers of completely different rock.
There was other evidence of continual modification of the land. You could see large areas of Scotland that had been scraped by glaciers, and big rocks sitting where there were no other similar rocks. It was obvious that huge glaciers or ice sheets had been there, but way before humans arrived.
There were places where you could see evidence of land being uplifted and subsiding.
In short, there were just too may examples which simply could not be explained in terms of one huge cataclysm. By about 1820 most educated people, including the religious establishment, accepted that Genesis could not be literally true, either in its timeline, or in its explanation of geology in terms of Noah's flood.