According to those who say Noah's flood was an actual global event, Where did the flood waters go when they subsided? Myself and others have researched this and could not answer it honestly and completely. We have thought that it went to the middle of the earth but there is only metal according to research.
Creationists who believe the flood was a real global event say the water is still here, in the earth's oceans.
The oceans are much deeper than the mountains of earth are high, and they cover a much greater proportion of the earth's surface. Some have estimated that if the earth's surface was flattened the oceans' waters would cover the land to a depth of 2.7km.
Creationists see passages like Psalm 104:5-9 as teaching that during the flood there was significant tectonic activity, as the mountains rose and valleys sank, giving us the continents we see today, and 'hiding' the flood waters in the oceans.
Ps 104:5 He established the earth on its foundations;
it will never be shaken.
6 You covered it with the deep
as if it were a garment;
the water stood above the mountains.
7 At your rebuke the water fled;
at the sound of your thunder they hurried away—
8 mountains rose and valleys sank—
to the place you established for them.
9 You set a boundary they cannot cross;
they will never cover the earth again.
Passages like these are ambiguous, and could be read as referring either to the original creation of land in Genesis 1:9-10, or to the aftermath of Noah's flood. In the case of Psalm 104 I think verse 9 does indicate that, at least by the end of this stanza, it is referring to the flood, for it speaks of God's promise not to flood the earth again, a promise that God had not made before the flood. I think the creation-flood ambiguity is deliberate - the story of Noah is a story of re-creation: a new earth, a new set of life (preserved on the ark), and a new family to repopulate the earth.
The water would for the most part go back to where it came from, which is the fountains of the deep. Gen_7:11 In the six hundredth year of Noah's life, in the second month, the seventeenth day of the month, the same day were all the fountains of the great deep broken up, and the windows of heaven were opened. So to explain, if the Earth's core heated up the water in the earth would start to boil and rise. This is what Jewish tradition tells us...the water from below was boiling. Once the heat reduced the water would want to go back to the lowest point. Another possibility is that the high mountains began to form. Since the mass of the Earth is constant any upshift would be offset by a depression. The rising mountains would create a loss of mass below the surface and the seabed (or some land area) would sink or a cavity would be created. If a cavity it could fill with water. There is more than enough water to cover the entire planet if the mountains were lower in the past. This would also mean the sea was shallower, which is what I hear from evolutionists all the time regarding ancient marine organisms.
The bible flood story is so much corroborated with other myths of a flood that it can be interpreted secularly as involving water coming down on earth.
The most likely thing to have happened is that atmospheric water began condensating due to a cosmic event that caused heavy raining. No rainbow prior to the flood means no condensation of steam in the atmosphere. Ionizing radiation or dust from space much have been lower to allow such a high level of steam in the atmosphere.
The resulting water must thus have ended up in the oceans or in ice mass. Increase in ice can be assumed since the volume of the atmosphere decreased as a result of being depleted of steam. Smaller atmosphere leads to lower pressure and thus colder climate, in accordance with the adiabatic lapse rate in the atmosphere.
I suggest asking in a physics forum to assess if the water ended up in oceans or ice.
The statement of God not to flood Earth again is not a promise. It is a consequence commentary that Earth cannot be flooded again. Water would then have to return to the atmosphere which it cannot do.
This question is much more a physics question than a Christianity question, although the Christian website Answers in Genesis has lot of investigations of various kinds on the flood event. Please also check there.