These caves have been documented for about 150 years though exploration was difficult due to the terrain and access. The Mulu Caves Project is an absolute goldmine of information and exploration history of the various caves. The photos (also from their site) here are of Deer Cave, dubbed the largest cave passage on earth. Very impressive and overwhelming.
What is even more amazing to me is that these caves are open to the public (akin to Carlsbad Caverns) with guided walkways above the floor. In the evening flights of millions of bats fill the sky in inky, black streams of thunder to feed in the distant areas. Below is a shot of the terrain and easily explains why it took so long for exploration to be possible.
Source - Mulu National Park
The famous pinnacles rise above the tree tops (over 45m) with their razor-sharp tips at Gg. Api and Benarat. In the limestone beneath the jungle canopy lie some of the world's most impressive caves. It includes the world's largest cave passage (Deer Cave), the world's largest natural chamber (Sarawak Chamber), and at 110km, the longest cave in South East Asia (Clearwater Cave). Just for the record, Sarawak Chamber can accommodate 40 Boeing 747 aircraft with room to spare and Deer Cave can hold England's St. Paul's Cathedral five times over. To date, about 195km of cave passage has been surveyed and more is expected to be revealed.