Tallest mountain under the ocean

The peak of Mauna Kea is 13,803 feet (4,207 m) above mean sea level but 33,476 feet (10,203 m) above its base on the floor of the Pacific Ocean. It is the world's tallest mountain by this measure, taller than Mount Everest 29,029 feet (8,848 m) which is the highest mountain above sea level.