Scientist believe that the Grand Canyon was onced covered by an ocean.

Yes, that is true. Geological evidence suggests that the Grand Canyon area was once covered by an ancient ocean, specifically during the late Paleozoic era, around 300 million years ago. The rocks found in the Grand Canyon contain marine sedimentary layers, indicating that the region was submerged under a sea at that time. Over millions of years, geological processes, including tectonic activity and erosion, led to the formation of the Grand Canyon as we see it today.