Designated a Unesco World Heritage site in 2003, the remarkable Phong Nha-Ke Bang National Park contains the oldest karst mountains in Asia, formed approximately 400 million years ago. Riddled with hundreds of cave systems – many of extraordinary scale and length – and spectacular underground rivers. Boats cruise along past buffalo, limestone peaks and church steeples to the cave’s gaping mouth. The engine is then cut and the boats are negotiated silently through the cavern. Visiting this NP was probably the highlight of our Vietnam holiday.