The Phong Nha - Ke Bang National Park - the most spectacular national park with some of the oldest and most complex limestone karst landscape in Asia. This 900-square-kilometer-area, largely covered by tropical forest, has extensive cave systems with hundreds of individual grottos, long underground labyrinths, waterfalls running beneath limestone giants towering toward the sky. Many rare species of animals and plants also call this park home. 112 species of mammals alone can be found here, including at least nine species of monkeys, such as macaques, langurs and gibbons.