When measured from its base on the ocean floor to its summit, the world's tallest mountain is Mauna Kea in Hawaii. Only about 4,207 meters of the volcano rises above sea level, but a much larger portion lies underwater.
Mauna Kea extends roughly 6,000 meters below the Pacific's surface and reaches 4,207.3 meters above it. Together, those sections give the mountain a base-to-peak height of approximately 10,200 meters, or more than 33,000 feet—greater than Mount Everest's height above sea level.
Mauna Kea is a dormant shield volcano on Hawaii's Big Island. It was built by repeated eruptions as the Pacific Plate moved over the Hawaiian hot spot. Its summit is unusually dry and high, making it one of the world's premier astronomical observing locations; several major observatories operate there.
The key distinction is between tallest and highest. Mount Everest is Earth's highest summit above sea level, while Mauna Kea wins when the measurement begins at the submarine base. The exact total varies because a volcano's geological base is difficult to define precisely.