A microbial world in stone
Hwanseongul Cave is a vast limestone system formed an estimated 530 million years ago. Its total known length is 6.2 kilometers, with passages that can reach 100 meters in height. While the cave contains typical speleothems like stalactites and stalagmites, its most unusual features are the soft, white moonmilk deposits. Unlike hard, slowly dripping calcite formations, moonmilk is a microcrystalline aggregate with a paste-like consistency, primarily composed of calcite (CaCO₃).
The formation of moonmilk is an active process of biomineralization. The cave's microbial communities, particularly filamentous bacteria from the phylum Actinobacteria, are important in its creation. These microorganisms influence the local water chemistry through their metabolic processes, such as ammonification, which increases the environmental pH. This change causes dissolved calcium carbonate in the water to precipitate into fine, needle-like crystals that are 50 to 500 nanometers wide. The bacterial cell walls themselves can be nucleation sites, providing a template for the calcite crystals to form upon. This direct biological involvement means the cave itself is generating new types of rock formations.