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Which mountain is the tallest in the world? That depends on how you measure them.

There are two mountains that could claim the title of the world's tallest. The summit of Mount Everest is higher above sea level than the summit of any other mountain, but Mauna Kea is the tallest when measured from base to summit.

Mount Everest

When measuring mountain summits by their height above sea level, Mount Everest is the highest mountain in the world. Standing at 29,035 feet (8,850 meters) it reigns over the "eight-thousanders" club, a list that consists of the 14 mountains on Earth that are at least 8,000 meters tall.

Located on the border of Nepal and Tibet in the Himalayan mountain range, Mount Everest's peak was first climbed in 1953 by mountaineers Edmund Hillary and Tenzing Norgay during a British expedition.

Although Everest's original Tibetan name is Chomolangma, the mountain was given its official English name in 1865 by Andrew Waugh, a British Surveyor General for the Royal Geographical Society who was unaware of its local name because Nepal and Tibet were closed to foreigners at the time.

The youngest person to reach Mount Everest's summit was fifteen-year-old Ming Kipa, a Nepalese Sherpa girl. Because Nepalese law forbids people younger than 16 to climb Everest, Ming Kipa scaled Everest from the Tibetan side.

The standard route to Everest's summit is considered by climbers to be less technically difficult than other eight-thousanders. Therefore, Everest remains a popular tourist spot, despite the Nepalese government's requirement that all prospective climbers obtain a permit, which can cost each person upwards of $25,000.

For those who do not have $25,000 set aside for an Everest expedition, NASA has created an interactive tour of the mountain called Find Mt. Everest From Space, which was made using imaging technology from the Earth Sciences and Image Analysis Laboratory at the Johnson Space Center.

Mauna Kea

Mauna Kea, an inactive volcano in Hawaii, is the world's tallest mountain as measured from its base, deep in the Pacific Ocean, to its summit. The discussion over what counts as Earth's tallest mountain stems from the fact that a large amount of Hawaii's Mauna Kea is underwater, and therefore measuring it from above sea level does not do justice to its massive size.

Mauna Kea's summit is at 13,796 feet (4,205 meters) above sea level, but it extends about 19,700 feet (6000 meters) below the water's surface. Therefore, its total height is 33,500 feet (10,210 meters), nearly a mile taller than Mount Everest, according to the United States Geological Survey (USGS).

In Hawaiian, Mauna Kea means "White Mountain," and its snowy summit attracts skiers and snowboarders. Its lower slopes are popular areas for hunting, hiking, sightseeing and bird watching.

Its location near the equator makes Mauna Kea an excellent astronomical observation site. Low humidity and clear skies make the weather conditions near the volcano's summit nearly perfect for many of the world's leading space observatories. The mountain's distance from city lights and the island's strict lighting ordinance also help to provide the atmospheric clarity that telescopes require.

Mauna Kea is currently home to 13 telescopes , including NASA's Infrared Telescope Facility, Caltech's Submillimeter Observatory and Japan's Subaru Telescope, as well as the Keck Interferometer telescope, the largest single-mirror telescope in the world. The Mauna Kea Weather Center's web cams allow for online viewers to witness these stellar observatories in action.