Question: If mountain climbers need oxygen tanks to climb Mount Everest, how were Noah, his family, and the animals able to breathe on the Ark when they were above the mountains (‘ … and the mountains were covered.’, Genesis 7:20)?
Answer: This question presupposes that Mount Everest was the height it is now (8,848 m = 29,028 ft), and that the air pressure would not have changed at that height above normal sea level with the addition of the flood water.
Mount Everest was not the height it is now during the Flood. Earth’s highest mountains have fossils of sea creatures at their tops, showing they were once under the sea. The possibilities are that the sea rose to cover the mountains, or the mountains were once under the sea and have since risen out of the sea, or a combination of the two.
Many creationist scientists think that mountains such as the Himalayas were probably built by catastrophic movement of the earth’s continental plates during and after the Flood (see Q&A: Plate Tectonics). Measurements indicate that the Himalayas are still rising). The rate of rise now measured is just the remnant of the processes that occurred much faster in the past.
Mountain building occurred as a part of the geologic processes that deepened the oceans to take the waters off the land towards the end of the Flood. Some mountains could have existed before the Flood, but none like the current Himalayas, Alps, or Andes in height. In any case, there is only enough water on all the earth to cover mountains about 3 km (2 miles) high, if all the ocean basins were raised. So, if the waters were not 9 km deep, but much less, the question is no longer an issue.