Puerto Princesa Underground River, Puerto Princesa Subterranean River National Park, Philippines

Float down this underground river on a raft of superlatives. The Puerto Princesa Underground River in the Philippines is not only reputed to be the longest navigable underground river in the world, but it also flows through one of the world's most impressive cave systems. This UNESCO World Heritage site was just voted one of the New7 Wonders of Nature for its "mountain-to-sea" ecosystem that features old-growth forests, caverns full of remarkable rock formations, and the tidally influenced river that empties into the South China Sea. In order to preserve the cave, all visitors must secure a permit before entry. There's a limit to the number of people who can visit daily, so be sure to book a spot on a tour in advance.