Lang & Deer Caves - Mulu National Park
Whilst the Lang Cave is the smaller of the two, within you will find remarkable stalagmites and stalactites, which are beautifully lit.
The Deer Cave is larger, in fact so much so, that is it is the world's largest cave passage that can be accessed by tourists. The cave is over two kilometers long, and at points reaches over 170 meters high. The Deer Cave's other claim to fame is that it is home to more species of bat than any other cave on the planet. Across 12 species, it is estimated that the bat population in the cave numbers between two to three million.
This then leads on to potentially the most dramatic part of the day. Head to the bat observation area as sunset draws in, and watch the millions of bats swoop from the cave, spiralling up into the sky in a long trail. Due to the bats' use of echo-location to find their food, they tend not to come out if it is raining. However you will still be able to see them (and certainly be able to smell them!) within the caves.