Limestone is originally laid down beneath the sea as an accretion of fine material principally derived from shells and corals. Under intense pressure these sediments become rock, and with uplift of the land the limestone is subjected to the erosive forces of wind, rain, and stream.

There is a string of limestone hills between Trenggan River and Kenyam River, the largest Gua Besar visible from Teresik Hill. Close to Kuala Tahan is the limestone cave, Gua Telinga. Further south, in the catchment of Sungai Ruil, is Gua Landak, first explored only in 1986; and further west are outcrops such as Gua Tumpat, Gua Siput, and Gua Cemara, none as yet easily accessible to visitors.

In many cases, these caves are home to bats, swiftlets, snakes, toads, cockroaches, spiders, centipedes... In spite of this, or because of it, some people like to explore caves; and Taman Negara offers some outstanding opportunities.