Answer:
The mountains rose, the valleys sank down to the place that you appointed for them (Psalm 104:8).
Genesis 8:21, 9:11; 2 Peter 2:5
Did you know that if the mountains and oceans were leveled off there would be enough water on the earth right now to cover it to a depth of about two miles? Some Bible scholars believe that our Bible verse here describes how God ended the Flood. The mountains rose up and the Flood waters went down where the ocean basins formed. Even evolutionists talk about the mountains being raised up like this, but they believe it happened over a long, long time. We believe that God did it very quickly at the end of the Flood. (By the way, that’s why we find so many sea fossils near the top of mountains. Makes sense, doesn’t it?) So, where is the water? I believe it is all still here. Did you realize that three-fourths of the earth is still covered with water?! The next time you go to the beach and watch God’s powerful ocean, remember you’re looking at the waters that once covered the entire earth during the time of the Flood!
Source: https://answersingenesis.org/kids/noahs-ark/where-did-all-the-water-go-after-the-flood/