Situated in the Gomantong Forest Reserve, the caves and the surrounding area are a protected area for wildlife, especially orangutans. For centuries, the caves have been renowned for their valuable edible swiftlet nests, which are harvested for bird's nest soup. The most valuable of the nests, the white ones, can sell for very high prices. 

The birds' nest collection is an ancient tradition, and the trading of these nests has been done since at least AD 500. Twice a year, from February to April and July to September, locals with licenses climb to the roof of the caves, using only rattan ladders, ropes, and bamboo poles, and collect the nests. The first collection takes place early in the breeding season before the swiftlets lay their eggs. The birds then make another nest in which they finally lay their eggs. After the young have fledged, the second collection is made. 

There are two cave complexes ? Simud Hitam (Black Cave) soaring up to 90 metres high and the more accessible of the two, and Simud Putih (White Cave), where the more valuable nests are found. You will need to make prior arrangements for exploring the more challenging Simud Putih which calls for more serious caving. Simud Hitam opens its door to the public and is a five-minute walk from the registration centre. A well-maintained boardwalk gives you easy access to do some exploring or watch the skilled harvesters at work.

Although the smell coming from the cave can be unbearable, we do recommend to stick around in the evening around 6pm to anticipate the bat exodus. Millions of bats will come out hunting and the dancing-like display in the air is quite an entertaining sight.