Final answer:
The United States has the third highest volcano in the world, Mauna Kea in Hawaii, which is measured from the ocean floor to its summit.
Step-by-step explanation:
The country with the third highest volcano in the world from its base on the ocean floor to its tip is the United States. Specifically, this refers to the Hawaiian Islands, where massive shield volcanoes have formed due to a mantle hot spot beneath the Pacific Plate.
Among these immense volcanic mountains is Mauna Kea, which is a now-dormant volcano that, when measured from its underwater base to its summit, is taller than Mount Everest, standing at 13,796 feet above sea level.
This makes Mauna Kea not just a significant geological feature, but also one of the world's premier locations for astronomy due to its high altitude and clear skies.