It's a common mistake that people assume Mount Everest as the tallest mountain in the world.
Mount Everest has a height of 8,848.86 m from its base to summit. While Mauna kea surpass it.
Mauna Kea is a inactive volcanic mountain in the Hawaiian Island in Pacific ocean . Mauna Kea has a height of 10,205 m from its base.
If we talk sheer about height then , Mauna kea is the tallest mountain. But it is half submerged in the pacific ocean. Mauna Kea is nearly a mile taller than Everest.
Mauna Kea is an esteemed, sacred site within Hawaiian culture. Mauna Kea is about million years old . Scientist estimated that it last erupted around 4,600 years ago.
Now this mountain has become a paradise of geographers and scientist who collects data of the clear sky and different celestial bodies.
source :Google
It all depends how we measure them. Technically in terms of height Mount Mauna kea beats Mt. Everest.