Today, I saw one of the coolest things I have ever seen. I went to Tham Kong Lor, which is a river running 7km through a cave, taking you clear under a mountain. If you ever plan to visit here, stop reading because the uncertainty of what to expect is half the fun.
For starters, here is the mountain that we passed under. Justing thinking about how much rock was over our heads as we cruised down the river was mind boggling. You can see the cave entrance near the waterline.

The water in this river is crystal-clear, unlike the Mekong, and refreshingly cold since it spends so much time out of the sun as it passes through the cave.
At the entrance, you can see well enough for pictures. Note the huge boulders just hanging from the ceiling. The cave opening at this point is probably about 30-40 feet tall.
But just a hundred feet or so into the cave, it starts to get really dark. The only light you have as you pass through the tunnel is the driver's headlamp, but when he turns his head away to check the motor, you find yourself in pitch black darkness, your eyes straining to see nothing. I also had a flashlight with me, but chose to only turn it on when I caught a glimpse of something that looked interesting, preferring to enjoy the atmosphere of the relative darkness.
Shortly after entering the darkness, the boat motors upstream into another "room" of the cave, in which the ceiling just vaults up. It is like entering a giant cathedral. It is such a humbling feeling -- you feel just so small and weak in the presence of this great mountain.
At several points along the way, it is necessary to disembark and wade through the river, while the boatman hauls the boat over small rapids and shallow water. This is not so difficult for the tourists, but is a pain for the boats carrying cargo through the tunnel, since they have to unload and reload heavy sacks at each of these points. At one of these areas, we were led up a slippery rock slope to view some of the cave's rock formations. Throughout the cave, there are drips falling from the ceiling, slowing creating the typical stalagmites and stalactites...

... but it was these curtains of rock that I loved. They even make a metalic sound when you whack them with your hand.

The river continues through many bends, just as any "outdoor" river would. At one point, we passed under an arch and around one of these curves to find ourselves in a long, straight, narrow passageway, with a boat coming in the opposite direction. It was an odd feeling, like being in a roadway tunnel, but on the water. It really enhanced the already existing feeling that we were somehow secretly "sneaking in the backdoor".
A few minutes later, we turned a corner and I thought I saw some light. It is hard to tell, though, because sometimes it is just the driver's headlamp reflecting off a light-colored rock. This time, it turned out to be the upstream exit, where we found the water much warmer as it flowed into the cave. A few minutes upstream from the cave, we parked the boat at the edge of the river and got out to stretch our legs before starting the journey back down the river.

The journey had taken more than an hour and it was nice to have a little time to reflect with awe on the wonder we had just experienced.

The trip back was, naturally, faster since it was downstream. This also meant we didn't have to disembark quite so often. For the tail-end of the journey back, I got to ride in the front of the boat, which holds a completely different experience. With no one sitting in front of you, you almost get the feeling that you are the first person discovering an untouched cave. And even though the water is flowing, it is slow enough that you get perfect reflections of the ceiling on the water, making it seem like you can see clear through the water to the bottom of the riverbed.
It was an incredible experience that was over too soon. And to think, I almost didn't make the journey out here because it consumed three days. What a mistake that would have been.
I know for certain that there are similar caves in the US and in Central America. I am sure there must also be others throughout the world. If you have the opportunity to see a cave river like this, don't pass it up.