My first trip to Mulu National Park was in the early 90s. I can't remember the exact year now, but it was an amazing experience visiting some of the biggest caves in the World. The locals have known about the Mulu Caves for eons - in fact the Penans and the Berawan tribes live in the jungles in the Mulu area, but serious explorations only started in the 1930s, followed by the extensive Royal Geographical Society Mulu Expedition in 1977. The caves are now a Unesco World Heritage Site.
After a lapse of over 20 years, I finally made it back to Mulu National Park in 2015. 20 years ago the Park was a very basic set up. Now it is a well-oiled machine run by the Sarawak Forestry Corporation. There is a very modern Park office where you can book the guided tours, and there are also accommodations and dining facilities on site.
There are many caves open to visitors - some are easily accessed by almost anyone, whereas others require moderate to serious fitness and agility. All visits to the caves must be accompanied by trained guides. There are also several jungle treks which can be explored without a guide.
Accommodation at the National Park can be difficult to come by - you need to book months in advance. Luckily there are many private lodging houses within the vicinity of the Park.
Getting to Mulu used to be quite tricky. There were only infrequent flights with the Twin Otter from Miri - and on my first trip we had to take a 5 hour boat trip in the wee hours of the morning from Mulu to Marudi to get back. That was quite an experience - reminded me of the boat trip on Apocalypse Now. Now it is a lot more convenient, as the airport has been upgraded to receive larger aircrafts. MASwings operate daily flights from Kuching or Miri using the ATR 72-600 Twin-Prop aircraft.