I realize that this topic has been addressed here and there in many threads. I'd like to collect all the points into one clearly labeled thread. Let's list all the arguments and evidence against the flood. They can include, for example, geological evidence, or internal problems with the Genesis account.
I'll start.
If there had been a global flood, there would be a single layer of silt extending all around the world. No such layer of silt exists.
This is a list of points and questions that I have thought about over the years. It is not complete, but just what I could recall quickly.
1. Not enough water available to cover the tallest mountains plus ≥15 cubits.
How would the location of that mountain be known to do a sounding?
2. Rain for 40 days and nights in the requisite volume would increase the pressure and temperature of the Earth to the point that it would cook the Earth, the ark, its crew and cargo.
3. How would the animals get to Noah and return home?
The logistics of animals arriving from around the globe to a single location in the Middle East in seven days can't be carried out today with modern transportation.
4. There is no global flood layer.
A flood with the scope and ferocity to cover the Earth in 40 days would produce a large, mixed layer of debris that would be obvious and global. None exists.
The existing fossil beds as evidence of a flood make no sense. Different ages. Different groups.
The permafrost megafauna carcasses as flood evidence makes no sense.
They are predominantly mammals that died over the course of 50,000 years and not in a single event at the same time.
5. There are parts of the world with no evidence of having had standing water or even rain in millions of years. The Atacama Desert of South America.
6. 8 people and 1 or 7 pairs of each species does not constitute a viable, basal population to re-establish the existing populations of living things.
Not enough diversity.
Potential of screening for recessive traits in subsequent generations. Reduced fitness.
7. No universal genetic bottleneck in global populations that is dated to the same time in all species.
8. No evidence that current geological features were carved in the wake of a giant flood.
The Grand Canyon was created by the gradual erosion of various layers of rock laid down over millions of years. Some of those layers are wind-formed between layers that are water-formed. How could any flood do that?
9. How would marine and terrestrial aquatic organisms survive in brackish, turbid, sediment-filled waters that rapidly went from raging to incredible depth over the course of a year?
Not all species of marine life live at depth. How would those that don't survive?
How would terrestrial freshwater species survive?
10. As a group, plants would not survive covered in water and sediment for a year.
Experiments show that an olive tree cannot survive even a few months entirely covered in water, let alone retain leaves.
11. The ark would not be large enough to house and maintain 10 million plus animals.
12. Eight people would not be enough to care for 10 million plus animals.
13. There would not be enough space on the ark to provide food for 10 million plus animals.
14. What is a pair of any eusocial organisms? Is it two bees or two colonies. Two termites or two colonies.
15. How would you save all the species of termites, timberworms, powderpost beetles, capenter moths, wood boring beetles, etc. on a wooden boat for a year?