Where did the water come from?
We know that a very large amount of water cascaded from the sky, some think this might have been a canopy of water God put in the atmosphere described in Genesis 1:6-7. This is very likely.
Also, God says that “all the fountains of the great deep [were] broken up”. This would be talking about volcanic action underneath the water. This would’ve caused great tsunamis hundreds of times more devastating then any major tsunami we have heard of.
But, not only that, these “fountains of the great deep” would also be referring to underground rivers and lakes. This would’ve contributed greatly to the flood and the 40 days and nights of rain helped as well.