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A universe underground © David A Knight/Shutterstock
When Vietnamese farmer Hồ Khanh stumbled upon this cave in 1991, it was immediately clear the gaping mouth led to a huge, dark, untouched chamber, complete with a free-flowing underground river. What couldn"t have been apparent to him then was that this cave, now known as Sơn Đoòng, is by far the world"s largest by volume.