Dark Cave, because it is made of mostly basalt in addition to limestone. The basalt is a black hard rock, giving it its dark name. Basalt and limestone surrounding you, you squished through the passage using only your headlamps.
By the time we got through the passage, you are covered in mud and ready for another swim. This time you can go further into the cave, where the width was probably 100m and the depth is deeper than anyone could touch. As you swim, you have to duck under huge stalagmites and balance between limestone columns to get to the next big opening.
One the way back, everyone turn on their lights during the long swim stretch and you can swim in darkness. It is surreal. Probably the coolest thing you haven’t done.