The Bible says that the water was deep enough to cover the highest mountains to a depth of 15 cubits. Since the flood occurred
after the 6 days of creation, this means that the topography of the Earth must have been essentially the same as it is today. Mt. Everest is about 8850 meters above sea level. Even allowing for the climate to have been warm enough that there weren't any glaciers or ice sheets, Mt. Everest still would have been nearly 8800 meters above sea level if the amount of water was the same as it is today. Covering the entire Earth with 8800 meters of water would require more than double the amount of water than is currently on the Earth. What happened to all the water that has vanished?
Covering the Earth with 8800 meters of water in 40 days would have required rainfall rates averaging more than
350 inches per hour over the entire Earth.