The rocks that grow
The Sangkulirang-Mangkalihat Peninsula in Borneo is a landscape of towering limestone cliffs and deep caves, covering an area of 1.8 million hectares. This is not ordinary limestone. The karst here is a biokarst, meaning the rock itself is actively built by living organisms. In the humid, tropical environment, complex microbial communities, thick biofilms of cyanobacteria, algae, and other bacteria—thrive on the rock surfaces. Microbes facilitate the precipitation of calcium carbonate from the water that seeps through the jungle, adding microscopic new layers to the cave walls and formations. This process of biogenesis means the caves are, in a sense, alive and still growing.
This microbial activity is a fundamental part of what makes the region's geology unique. The Mangkalihat caves are formed by the dissolution of rock by water, which is typical for karst landscapes, the Mangkalihat caves are also shaped by this additive, biological process. The thick mats of microorganisms create their own micro-environment, trapping nutrients and influencing the flow of water, which in turn affects the structure of the caves over geological time. The result is a vast and complex system of hundreds of caves with varied passage shapes, from angular corridors following fault lines to rounded, water-carved tunnels.
A Pleistocene art gallery
Deep within these living caves, such as Lubang Jeriji Saléh, are galleries of prehistoric rock art. In 2018, researchers dated a large, reddish-orange painting of a wild bovid, likely a Bornean banteng (Bos javanicus), to be at least 40,000 years old. This discovery identified it as the oldest known example of figurative art anywhere in the world at the time. The dating was achieved using a technique called uranium-series analysis. This method measures the radioactive decay of uranium in the thin calcite crusts that have formed over the top of the ochre paintings. This provides a minimum age for the artwork underneath.
The art in these caves appears in at least three distinct phases. The oldest phase, from 52,000 to 40,000 years ago, includes the large animal paintings and reddish-orange hand stencils. A later style, dated to around 20,000 to 21,000 years ago, features mulberry-colored hand stencils, sometimes filled with complex line and dot patterns. The most recent phase includes human-like figures in black pigments, one of which has a minimum age of 13,600 years. The presence of this art, created at the same time similar artistic traditions were emerging in Europe, suggests that the capacity for symbolic and artistic expression developed across the globe, in many locations.