The tallest mountain isn’t Everest
At 10,210 metres Mount Mauna Kea in Hawaii is the world’s tallest mountain. Mount Everest is 8,848m tall. Surveyors typically measure the height of a mountain from its base.
Mount Mauna’s base extends well below the level of the sea. It is nearly a mile taller than Mount Everest, according to the United States Geological Survey (USGS).
Sources:
https://www.livescience.com/32594-which-mountain-is-the-tallest-in-the-world.html
https://en.wikipedia.org/wiki/Mauna_Kea
https://geology.com/records/highest-mountain-in-the-world.shtml