Of the sites, Phong Nha-Ke Bang National Park in the northcentral province of Quang Binh topped the list.

According to details given by the travel guide, the remarkable Phong Nha-Ke Bang National Park was designated as a UNESCO World Heritage Site in 2003 and contains the oldest karst mountains in Asia which were formed approximately 400 million years ago.

“Riddled with hundreds of cave systems – many of extraordinary scale and length – and spectacular underground rivers, Phong Nha is a speleologists’ heaven on earth, and a real treat for anyone who appreciates the sight of nature at its most raw,” it added.

The caves are the region's absolute highlights, although the above-ground attractions of forest trekking, the area's war history, and rural mountain biking means that the park has enough going on to warrant stays of up to three days.

Lonely Planet stressed that more than 100 types of mammals; 10 species of primate, tigers, elephants, and the saola; a rare Asian antelope; 81 types of reptile and amphibian; and more than 300 varieties of bird have been logged in Phong Nha.

Son Doong Cave ranked in second, followed by Tu Lan Cave and Paradise Cave.

Rounding off the list of the must-see attractions in northcentral Vietnam were En Cave, Van Long Nature Reserve, Toi Cave, Trang An, Cuc Phuong National Park, Phat Diem Cathedral, Mua Cave, and Bai Dinh Pagoda.