Mauna Kea, Hawaii

Mauna Kea is an inactive volcano, and one of five that make up the island of Hawaii (part of the state of Hawaii, USA). It is tall enough with a peak of 13,803ft above sea level, but starting at the base, which is deep under the ocean, the total height is over 30,000ft, which would make it the tallest mountain structure in the world. The name translates to 'White Mountain', referencing the snow at the summit, unusual for its location despite the height.