As I am a nature beauty lover, Phong Nha National Park in Quang Binh province easily becomes my top favourite destination. 

“Phong Nha-Kẻ Bàng National Park is a national park and UNESCO World Heritage Site in the North Central Coast region of Vietnam, about 500 km south of Hanoi, bordering the Hin Namno Reserve in Laos. Characterised by mountains, tropical forests and underground rivers, it’s known for its ancient limestone karsts and vast network of caves."

You must be surprised when I tell you that the biggest caves in the world is located here in the middle of Vietnam. These include Sơn Đoòng cave and Phong Nha cave.

"Hang Sơn Đoòng has an internal, fast-flowing subterranean river and the largest cross-section of any cave, worldwide, as of 2009, believed to be twice that of the next largest passage in Malaysia. It is the largest known cave passage in the world by volume. You can imagine a cave that is big enough to fit an entire New York City block complete with 40-storey skyscrapers.

The cave was formed in soluble limestone and is believed to be between 2 and 5 million years old"

Source: Phong Nha – Ke Bang National Park Headquarter website and Wikipedia.