If you believe that there was at some point a global flood that covered all the mountains: [BIBLE]Genesis 7:19[/BIBLE]
Then where did the water come from? If it used to be underground, then the crust would have to have a porosity of about 50%; in reality, we find that it is about 1% due to the extreme pressures (source: http://www.eric.ed.gov/ERICWebPortal/recordDetail?accno=EJ278607 ). If it came from the atmosphere, then the sheer enormity of the amount of water present beforehand would introduce similar problems, as well as raise the question of why it didn't rain beforehand.
Then where did the water come from? If it used to be underground, then the crust would have to have a porosity of about 50%; in reality, we find that it is about 1% due to the extreme pressures (source: http://www.eric.ed.gov/ERICWebPortal/recordDetail?accno=EJ278607 ). If it came from the atmosphere, then the sheer enormity of the amount of water present beforehand would introduce similar problems, as well as raise the question of why it didn't rain beforehand.