Maybe you are familiar with Halong Bay and its limestone islands that rise dramatically from the jade green sea. Most visitors only experience this from the deck of a boat, but the true magic begins when you explore these islands with a kayak. Some of islands are hollow and have secret lagoons inside, which are only accessible from narrow tunnels and caves that are filled water at high tide. Through the entrance tunnel, the lagoons will open up with jungle vegetation clinging to the steep walls and a silence hard to imagine (especially in Vietnam). With a good guide and a tide table, it's possible to explore this hidden side of Halong Bay, which is even more magnificent from the inside.