It's a small cave hidden in a jungle on the loop trail to 1000 year old tree. First grotto gets a lot of light from the outside and if you go deeper it gets totally dark so watch out where you step as there's a big whole at some point.
My favorite patk in Vietnam! There are many caves to explore and the landscape makes it unique. There are quite good roads so riding a motorbike is easy and pleasurable among the mountains. You have to visit this place!!