Gunung Mulu National Park is a World Heritage site. Even before it got this UNESCO listing, it was a popular tourist destination. Therefore many postcards have been printed over the years.
I will just show the cave related ones here.
Royal Mulu Resort 1 - scenes of Mulu
Royal Mulu Resort 2 - one of the spectacular entrances
Royal Mulu Resort 5 - the entrance to the massive Deer Cave
These 6 cards (above and below) are from the Royal Mulu Resort
Royal Mulu Resort 6 - the lush primordial entrance to the Deer Cave
Royal Mulu Resort - Lang Cave
Royal Mulu Resort - river kayaking & adventure caving are popular activities
Deer Cave -
Deer Cave, Abraham Lincoln, Wind Cave [Journey through Borneo 113]
Largest cave passage known to man [Scenes of Sarawak 705]