Everest wasn't in existence yet during Noah's time

I was reading Genesis 7:20 which states:

Fifteen cubits upward did the waters prevail ; and the mountains were covered.

One might be led to think that Everest was covered by that flood. A little research shows that Everest hasn't always been the tallest mountain.

Indian mathematician and surveyor Radhanath Sikdar was probably the first person to identify that Mount Everest (then known as peak XV) was the world's highest peak as he was the first person to calculate the height of the mountain in 1852. However it was officially announced in March 1856.
(Source, http://timesofindia.indiatimes.com/Hyderabad/Indian_mathematician_1st_to_identify_Everest_as_highest_mountain_peak/articleshow/3091546.cms)

Mount Everest lies between Tibet and Nepal in southern Asia. It is 9 kilometers high. The waters from the worldwide Flood of Noah's time would cover a fairly level earth only to about 3 kilometers. So how could the water form Noah's Flood cover Mount Everest?

The answer is that Mount Everest was formed after or during the Flood — it wasn't in existence in its present form before the Flood. We know this because its higher parts contain fossils of sea creatures and seashells, showing that it is made of rock that was once under water.

Before the Flood, the mountains were not as high as today, and the ocean valleys were not as deep. Mount Everest and other high mountains were pushed up by the enormous underwater volcanic activity of the Flood (“the fountains of the great deep” — Genesis 7:11) and carved out by the rapid draining away of the waters into the ocean valleys.

So the water didn't have to cover Mount Everest and the other high mountains we see today, because they didn't exist before the Flood. There were hills in the pre-Flood world of course (Genesis 7:19), and it was these that the Bible speaks of when it says the mountains were covered to a depth of 15 cubits (6.75 meters) during the Flood.

(Source, http://www.creationtips.com/floodmount.html)