The question is, "Where did all the water come from, and where did it go?" The planet earth has more water on it and under its crust than all the other planets of our solar system. When earth was first created "in the beginning" it was covered with water (Genesis 1:2). On the second day of creation (Genesis 1:3), something happen to some of this water at the surface. A lot of water went below the crust of the earth and the dry land appeared. Could this underground subterranean water be what powered the "fountains of the great deep" (Genesis 7:11) during the Flood of Noah's time,
Just research the vast geologic features we see on the earth's surface and under the oceans. A global flood with its vast and unique consequences did occur. For example, coal, oil, and methane hydrates did not form over hundreds of millions of years; they formed in months. Fossils and layered strata did not form over billions of years; they formed in months. Our Grand Canyon did not form in millions of years; it formed in weeks. Major mountain ranges (Himalayas, Andes, Sierra Nevada, Rocky Mountains, etc.),did not form over hundreds of millions of years but in hours. These statements may appear very shocking to people who believe the earth is billions of years old.
But the worldwide flood in Noah's time gives us the best explanation about how the earth became to look like it does today. The cataclysmic flood waves, volcanic activity, earthquakes, and torrential rain falls caused by tremendous storms truly shaped the features on the earth in a short period of time with the rapid burial of plants and animals in sedimentary layers of sandstone, limestone, and other sediments to form the rock layers that we see today. It was a rapid process and did not take a long period of time to accomplish like evolution (Uniformitarianism) believes it did.
The eruption of Mount St. Helens on May 18, 1980 gives us a small example of what happened during the global flood. When Mount St. Helens erupted it produced mud flows that laid down enormous amounts of sediment, covering areas to pile up sediment to 600 feet thick in many places. Within five years they discovered some of this sediment turned into solid rock. Another eruption on March 19, 1982, produced more mud flows that eroded large canyons were the sediments piled up. These canyons, especially the one called "Little Grand Canyon" look like the Grand Canyon with its layered strata rocks. Creation scientists believe the Grand Canyon of Arizona was formed in the same fashion; the rock layers were laid down during the global cataclysmic Flood and volcanic upheaval of Noah's day (Genesis 6-8), and the canyon itself was carved in later by a post-flood catastrophe when Grand Lake was breached.
But where did the water go after the Flood? This water is mostly in the oceans of today (Pacific, Atlantic, Indian, etc.). Just look at the Pacific Ocean with its enormously deep trenches such as the Mariana Trench which is 6.86 miles deep. When the continents rose with their mountain ranges the water flowed into the ocean basins that were formed. This is where most of the water went. Some water is still under the crust of the earth.