Many have puzzled over that, since the volume of water in the oceans wouldn't seem to be sufficient to cover earth's present mountain ranges. But if they volume of water within earth and under the oceans is actually 3 or 4 times greater than the water on the surface, and if the mountains originally were not so high, but were pushed higher by tectonic forces---
I don't know.
That is not what Chapters 6, 7, and 8 had to say about the Flood. The Biblical account is consistent with the occurrence of a great regional flooding and tidal wave. A number of such great catastrophic events are known to have taken place in that part of the world in prehistoric and early historic times. In particular there were great floodings of the Mediterranean Basin, the Black Sea basin, and the Persian Gulf upwards through the Tigris-Euphrates river valleys, hills, and mountains.