Terry, Terry, Terry....
I have had it explained to me quite clearly by eminent WT Bible scholars that before the flood, the highest mountains were only a few thousand feet high.
The force of so many millions of tons of water caused the mountains to grow from a few thousand feet to 29,000+ feet, over the course of the year the earth was flooded.
Plate tectonics, shplate tectonics. This is all Bible-based.