Noah's flood must have been BIG - no matter whether it was regional or global.
Some argue that there isn't enough water on earth for it to be possible.
However, 70% of the earth's surface is already covered by ocean, to an average depth of nearly 4km.
And who knows how much water exists beneath the earth's crust. There could be a lot of water down there. 80% of what comes out of volcanos is water vapor. Scientists also have reason to believe that earth's outer core is liquid.
But could there be enough subterranean water to submerge Mt Everest by 15 cubits? Well keep in mind that if this amount of subterranean water exists, it would occupy a mere 0.0029044% of the earth's volume. So, relatively speaking, we are still not talking about an impossible volume of water. And since the deepest that mankind has drilled beneath the sea-bed so far is only 2,111m, I guess it can't be ruled-out.
But in all likelihood, the earth's mountains were not as high before the flood. Instead, we could say that the aftermath of the flood may have caused mountains such as Mt Ararat and the Himalayas to become as high as they are today.
According to a NASA website (NB:- this was a NASA website, not a Creation-Science website!) there are lots of marine fossils on top of Mt Everest and the terrain was once a seabed. And the Rocky's is the world's largest site for fossils. Apparently these marine animals were covered in sediment and then the seabed was forced upwards to form the Himalayas and the Rockys. What better explanation for this than the flood and its tectonic aftermath?
Notice the Bible says that the earth was divided in the fifth generation after Noah - possibly as a result of continental drift or rising sea levels. Apparently there were a lot of major changes that occurred on earth's surface for quite some time after the flood! These changes could have included shifting tectonic plates, continental drift, earthquakes, volcanos, rising mountain peaks and rising sea levels, and the cutting-off of land masses that were once connected.
So it's quite possible that before the flood, the mountains may have been much lower (Mt Everest and the Rocky's may not have even existed before the flood - they may have been seabeds!). And it's also possible that the world had only one large, connected land mass, one connected continent, before the earth was divided after the flood. The world's oceans contain enough water to completely submerge the earth to a depth of 2.7km if all the land was flat. If we allow for lower mountains and shallower oceans before the flood, we wouldn't have to find too much subterranean water at all, and all the world including all its mountains could have easily been submerged to a depth of 15 cubits.