This cave is so big that it could host a town with a few sky scrappers. It's 200 m high and 9 km long. You can see here the tallest stalagmites in the world, a cave river and even a forest. There are a couple of areas where the ceiling of the cave has collapsed, which allows sunlight to come through, just enough for the trees and other plants to grow. This natural wonder of Vietnam became available for public only since 2013. When you book your tour, make sure it's a legitimate operator that has a government permit. Also keep in mind that some parts of the cave may be flooded during the rainy season so the tours operate from February till August.