The tallest mountain on Earth isn't Everest — most of it is just underwater

Measured from sea level, Everest wins easily. But measured from base to summit, Hawaii's Mauna Kea is taller: it rises about 10,200 metres from the Pacific seafloor, though only around 4,200 metres of that poke above the waves, leaving Everest's 8,849 metres the tallest point above sea level.

— Wikipedia contributors, Mauna Kea — Elevation from base