Situated in the heart of Quang Binh Province in north-central Viet Nam, 500km south of Ha Noi, Phong Nha is "a fossil-rich site and a location where new caves are still being discovered", the magazine writes.

It notes that the entrance to the 44.5km, nine-cave system is via an underground river that connects with the Son River. Tour boats can penetrate inside to a distance of 1,500m.

"There are lots of trekking, kayaking and swimming opportunities locally too. Be sure to lookout for a multitude of beautiful butterflies if you are there in March, April or May."

Other caves in the list of top 5 "incredible explorable" caves are: Waitomo Cave (New Zealand), Indian Caverns (United States), Dan-yr-Ogof Cave (Wales), and Eisriesenwelt (Austria).

— VNS