Tha Khek, 350 km (217 miles) south of Vientiane, in Khammuan Province, is a bustling Mekong River port, with some of its ancient city wall still intact. It's surrounded by stunning countryside and karst (limestone caverns and sinkholes). There are some spectacular limestone caves in the area—notably Tham Khong Lor. This cave is more than 6½ km (4 miles) long, and is so large that the Nam Hin Bun River runs through it. Opposite Tha Khek is Thailand's provincial capital of Nakhon Phanom. Ferries ply the waters between the two cities.