An underground universe

By far the world’s largest cave by volume, Sơn Đoòng in Vietnam is big enough to fly a Boeing 747 through. The stalagmites rising from its floor stand 15 storeys high, its main chamber is 650ft (198m) tall, about 500ft (152m) wide and stretches for more than three miles. Inside one of its massive dolines - sunken, sun-exposed sections like that seen in our image - is an entire rainforest ecosystem, and that’s just what we know so far.
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