Scientists believe that the chain of underwater mountains beneath the Pacific which stretches away from Hawaii up to Alaska may have once included even more mountains. What happened to those moutain?

Those mountains are thought to have been built up by volcanic activity over millions of years. As the Pacific tectonic plate moved, the older mountains slowly sank back into the Earth's mantle, either through erosion or subduction at tectonic plate boundaries. This process is part of the natural cycle of plate tectonics.