The bases of mountain islands are below sea level, and given this consideration Mauna Kea (4,207 m (13,802 ft) above sea level) is the world 's tallest mountain and volcano, rising about 10,203 m (33,474 ft) from the Pacific Ocean floor.he bases of mountain islands are below sea level, and given this consideration Mauna Kea (4,207 m (13,802 ft) above sea level) is the world 's tallest mountain and volcano, rising about 10,203 m (33,474 ft) from the Pacific Ocean floor.