Gunung Mulu National Park is Sarawak's largest national park at 529 sq km. It lies just across the border with Brunei, some 50km south of Bandar Seri Begawan, Brunei's capital city, and approximately 100km east of Miri, the nearest Malaysian city.
Mulu features the Sarawak Chamber, which is the largest cave in the world, as well as some of Sarawak's best long-distance jungle trails. Among these is a 3-day trek to the Pinnacles, one of just three such stone formations in the world.
Mulu is tremendously popular among ecotourists despite the fact that it is practically only accessible by air (although a difficult journey by land and sea from Miri is also possible).
The remoteness of the park means that supplies can be expensive. Water from the tap is, according to park officials, not drinkable, though people have tried it with no ill effects. Bottled water is available at more than twice the price that it would fetch in a city. The budget traveler will find that some unimaginative variation of fried noodles will likely be his or her breakfast, lunch, and dinner for the duration of the stay, this being the cheapest thing available from the canteen.
A flashlight is a very useful thing to have, as both the jungle in the evening, and the caves at any time of day, can be pitch black. Visitors are warned not to use strong torches in the Deer Cave as this may disturb the bats; the unprepared will find torches of suitable strength (i.e. rather crap ones) at the little shop at the airstrip.
Caves around Camp HQ
More Adventurous Activities
Note that the following activities require that guides be hired, and possibly special equipment:
Sources: Lonely Planet, and my memory from the trip there.