Located in Gunung Mulu National Park, this vast network of natural limestone excavations includes a few of the biggest caves on the planet. Traveling to the caves puts you in the shoes of those who explored them initially, because they aren't easily accessible from the usual tourist tracks.
Your best bet would be to catch an adventure tour, which would take a longboat down the river from Miri (or the much quicker and more expensive charter plane ride) to the park. No matter your prior caving experience, these caves are magical. Deer Cave is the world's largest cavern, while the natural stalagmite posts from floor to ceiling in Drunken Forest Cave are incredible to experience firsthand. If that isn't enough, there is also Clearwater Cave, which is the largest on Earth by volume. Also, the main cave inhabitants are bats, naturally, and their mass exodus during feeding time is an awe-inspiring phenomenon that should be witnessed no matter where you choose to spelunk.
A tour of some of the show caves goes for around RM 20 ($6.50 US), although adventure tours will include more activities and cost more. Contact the official site for more information!
Note:Make sure that you book either a park tour guide or an adventure tour guide, because you won't be allowed in any of the caves otherwise. Accommodations are available in the area.