The mouth of Deer Cave in Gunung Mulu National Park (Borneo)
Deer Cave is a passage about 150 m (500 feet) in diameter and about 1 Km (0.6 Miles) long, opened on both end. It was, for many years, considered the largest single cave passage in the world.
The small silhouette at the bottom right of the image gives an idea if the size of this cave.
For more photos and information about Deer Cave, go to my Deer Cave photo series.