Mariner's Cave in Vava'u, Tonga, is an extraordinary underwater cave, perfect for experienced snorkellers who can dive about 4 m (13 ft) underwater to enter. Inside, you'll find an airtight chamber where the atmospheric pressure and temperature changes create an eerie fog. For scuba divers, there's also a lower entrance at 16.5 m (54 ft). This cave is not only a natural wonder but also a part of local folklore, making it a must-visit for adventurous travellers. Visit it for free if you're a yachtie. Otherwise, boat tours can bring you to this spot from Neiafu.