How tall is mountain?

Answer 1

Mountains are commonly measured by their height above mean sea level. Some mountains (like Mount Everest) actually rise from an elevated plateau and climbers do not start at sea level when climbing them. Others (on volcanic islands like Hawaii's Mauna Kea) extend from ocean floor bases thousands of feet below the surface. Generally, only undersea mountains, those that are completely submerged, are measured from base to summit.