The words "tall" and "high" are often used interchangeably to describe the height of an object, but there is a subtle difference between the two.
"Tall" refers to the height of an object relative to its base or surroundings. It is often used to describe human-made structures like buildings or towers. For example, the Eiffel Tower is tall because it stands high above the ground and is much taller than the surrounding buildings.
"High," on the other hand, refers to the distance of an object from the ground or sea level. It is often used to describe natural features like mountains or cliffs. For example, Mount Everest is high because it has the highest elevation above sea level compared to any other mountain in the world.
So to answer your question, Mt. Mauna Kea is the tallest mountain if you measure its height from its base on the ocean floor, but Mt. Everest is the highest mountain because it has the highest elevation above sea level.