Question :
What is the tallest mountain in the world?
Answer :
From the base to the summit, the volcano Mauna Kea in Hawaii is the tallest. Most of it just happens to be below water. Mt. Everest is the highest but not the tallest because it starts on the high Tibetan Plateau.
Almost everyone knows that Mount Everest is the highest mountain in the world and climbers from everywhere travel to Everest hoping to earn the distinction of climbing the "World's Highest".
The peak of Mount Everest is 8,848 meters (29,028 feet) above sea level. This high elevation gives Mount Everest the distinction of being the mountain with the highest altitude.
Mt.Everest
Mauna Kea has an altitude of 4,205 meters (13,796 feet) - much lower than Mount Everest. However, Mauna Kea is an island and if the distance from the bottom of the nearby ocean floor to the peak of the island is measured, then Mauna Kea is taller than Mount Everest.
Mauna Kea is over 10,000 meters tall compared to 8,848 meters for Mount Everest - making it the world's tallest mountain. 
Mt. Mauna Kea
Bonus Question :
Before Mount Everest was discovered, what was the highest mountain in the world?
Answer :
Mt. Everest was still the highest mountain even before it was discovered! Tricky, ha ha...
