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