Most travellers overlook the region, but there's plenty more to central Vietnam than Hoi An and Hue – and we've now got the trip to prove it. In addition to time spent in both of places, which are must-visits on any itinerary, this trip ventures where very few others do: up into the highlands of the Truong Son mountain range and underground in Phong Nha-Ke Bang National Park. Spend a night in the remote village of Bho Hoong, home of the ethnic Co Tu people, and learn about their traditional way of life. Then, after a couple of days exploring Hue's ancient citadel, get ready for something else completely different. Over two days and one night, venture below ground into one of the world's largest – and most recently discovered – cave complexes. Wind through narrow rock corridors, scramble along rocky outcrops, swim through subterranean rivers and camp in the jungle.