Khong Lor, a 7.5 km cave in Laos: I've been to the long cave in Palawan, Philippines, known as the "longest underground river in the world" and Khong Lor was 10 times better. It's more difficult to get to, which made it less crowded. The cave itself was quiet and calm, dark (duh), hearing only sounds of the boat paddling and water splashing. As you near the end of the cave, the guide speeds up, almost like he's spitting us out of the cave and into a beautiful jungle! If this is what it's like being born, I'm in!