There was enough water in the great deep to flood the Earth 1,200 km deep. 15 cubits was when the crew of the ark stopped taking soundings. Mount Everest formed 101 years AFTER Noah's flood during Peleg's tectonics.
How Deep was Noah’s Flood?
Uniformitarianism
People presume to disprove God by cobbling together some math based on Uniformitarianism ideals that were popularized by Charles Lyell, a contemporary of Charles Darwin.