What is the tallest mountain on Earth measured from its base on the ocean floor? Mauna Kea. From its submerged base in the Pacific to its summit, Mauna Kea rises roughly 10,000 metres or more, exceeding Mount Everest’s height above sea level.
Mauna Kea is a dormant shield volcano on Hawaiʻi Island. Only 4,207 metres of it stands above sea level, but much of the volcano is hidden beneath the Pacific. Its underwater flanks descend toward the Hawaiian Trough, making the mountain far taller when measured from base to summit.
Mount Everest remains Earth’s highest mountain above sea level, at 8,848.86 metres. That is why “highest” and “tallest” produce different answers. Mauna Kea’s exact base is also debated: depending on whether the measurement begins at the seabed or deeper geological roots, estimates vary.
Mauna Kea formed over roughly a million years as the Pacific Plate moved across the Hawaiian hotspot. Its summit is also famous for astronomy because the high, dry air provides exceptionally clear observing conditions.