A visit to this national park in Quang Binh Province -- which has been designated as a UNESCO World Heritage Site -- makes for a good day trip from Vinh. Famous for its massive grottoes, the Phong Nha cave system is 40km (25 miles) long, starting from Ruc Ca Roong Cave to Vom Cave, descending some 300m (984 ft.) before the cave water spills into the sea. The main grotto, and the highlight of a tour here, has 14 "rooms" covering over a kilometer-long (half-mile) corridor. Boats travel its length upstream. Contact tour operators in Hue or Hanoi to visit this stunning natural site. There's no public transportation to the caves.