Hang Son Doong Cave is the world's largest cave, located in Quang Binh province, Vietnam. It was found by a local man in 1991 and was explored in 2009 by British cavers. The name "Son Doong" cave means "mountain river cave." It was formed 2-5 million years ago by river water eroding away the limestone underneath the mountain where the limestone was weak. (Photo/DFIC)
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