White stalactites hang down from every surface, and the walls and floors are glazed with thick ice. The Ningwu ice cave, also referred to as Wannian Ice Cave in Ningwu County, China, is a unique phenomenon: The subterranean space stays frozen all year long, a ‘natural freezer’ despite the sometimes 70F (21C) summer temperatures outside the cave. How? From Science News:

The 3-million-year old Ningwu ice cave in China’s Shanxi province contains a single entrance connected to the top of a bowling-pin-shaped chamber. Geologists Shaohua Yang and Yaolin Shi of the University of Chinese Academy of Sciences in Beijing digitally re-created the 85-meter-deep formation and found that buoyant, warm outside air doesn’t flow very deep into the cave. Winter’s cold air, meantime, flushes heat out of the cave system. This convection process maintains freezing temperatures year-round even as a thousand visitors explore the cave each day.

The colors in the cave come from 200 light bulbs that illuminate the frozen water for the May to October throng of tourists. In the study, the geologists considered their presence, as well as the growing influence of climate change on the temperatures surrounding the ice.

Next: Inside the Svalbard Seed Vault, Ice drumming on Lake Baikal, and why ice cubes crack in drinks. Bonus: More videos about caves.

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