Son Doong Cave
I recently read about the biggest known cave in the world and how it was discovered. It turns out that Son Doong Cave in Vietnam was only discovered 23 years ago in 1991 by a local man named Ho Khanh, but not even then did they explore it. Big drop to the cave and strange sounds coming from the cave scared locals and cave was left unexplored for another 18 years. Finally in 2009, group of cave researchers decided to conduct a survey and see what's actually there.
Scary sound coming from the cave turned out to be a underground river, therefor the name Son Doong, meaning mountain river cave. But there was a lot more to it than just the river as it turned out to be a underground heaven with it's own cave systems, lakes and even jungles. All of that for five kilometers, 150 meters wide and 200 meters high. Jungle was made possible by the skylight where some of the roof had fallen in centuries ago.
But other than it's beauty, there's a lot to research as well. Some of the cave walls and floors are covered with cave pearls, formed there by a dripping cave water, that are now about the size of a baseball. Also, around the cave waterfalls scientists have found plants that they have never seen before. And thanks to it's jungle, even monkeys and flying foxes live in a Garden of Edam, one of the parts in the cave system.
So if you happen to have 3-4 thousand dollars and want to see something magical, then be sure to visit this place. I know I would.