3. Caves
Found in north central Vietnam, the Son Doong Caves have a total measured volume of 38.5 million cubic metres. Some sections are up to 200m high and 150m wide — enough to fit in a whole New York City block. Despite their size, the caves' opening was only discovered in 1990, by a local man. He didn't go inside, walked back to the village, and promptly forgot where to find them again. It wasn't until 2009 that they were rediscovered and the first expedition entered this huge cave system, home to giant stalagmites and stalactites, an underground jungle and its own localised weather system. To go inside, you'll need to book a tour with Oxalis, the only operator licensed to enter.
4. Island
Greenland (Nope, Australia is regarded as a continent, not an island), with a land-mass of 2,166,086sq km and a population of more than 56,000.
5. Skyscraper
At 160 storeys and a total height of 829.8m, Dubai's Burj Khalifa mega-skyscraper is the world's tallest. The building also boasts the world's highest observation deck — its outdoor terrace is 555m above sea level.
6. Tree
Sequoias, which can grow up to 83m tall. The oldest in the world is in California's Giant Forest of Sequoia National Park, Tulare County, and is estimated to be 3500 years old. In comparison, New Zealand's Tane Mahuta, in Northland, is 51.2m tall and is estimated to be between 1250 and 2500 years old.
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