Sơn Doong has been open for visitors since 2013, four years after the British experts announced it as the world's largest cave with its own inner ecology. Sơn Đoòng is 150m wide, 200m high and 9km long, formed about 2-5 million years ago and had no human traces when it was found. Let's follow the trekkers to discover the hidden charm of the world's largest cave.