Imagine a cathedral-like inner chamber shot with splinters of sunlight and a cave floor swarming with cockroaches, and you have the Gomantong Caves. The smell has a presence of its own, thanks to the ubiquity of bird and bat guano (you'll want to wear covered shoes and a hat). The only cave open to visitors, Simud Hitam, is magnificent. The turn-off to Gomantong is well signposted en route to Sukau; most tours include a stop here.
A standard visit involves a trot counter-clockwise along the raised boardwalk that loops around the vast chamber. Besides cockroaches, you may spot freshwater cave crabs, giant centipedes, scorpions, plus lots of swiftlets and bats that occasionally fall to the cave floor and provide nourishment for the scuttling creatures that dwell in swiftlet and bat excrement. If you happen to be outside the cave entrance at around 6pm, you will see an impressive vortex of bats streaming out in search of food (unless it's raining, in which case they tend to stay in).
The majority of visitors to Gomantong come as part of an add-on to their Kinabatangan tour package. If you're driving the caves are reachable via 5km of paved road, around 22km northeast of Bilit and 27km northeast of Sukau.
Due to dwindling swiftlet bird populations, the caves are closed over certain periods, so check before planning your visit.