Mount Everest is not the tallest mountain on Earth when measured from base to peak. That title belongs to Mauna Kea in Hawaii.
Why it is true
Although Mount Everest has the highest elevation above sea level, Mauna Kea in Hawaii is taller when measured from its base on the ocean floor to its peak. Mauna Kea is over 10,000 meters tall in total, of which only 4,207 meters are above sea level.
Source: USGS
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