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.
The geology of Hwanseongul
Hwanseongul is the largest limestone cave in South Korea and was designated a Natural Monument in 1966. The total cave system is estimated to be over 8 kilometers long, with 1.6 kilometers of steel and iron walkways open for public viewing. Inside, the environment remains stable, with a year-round temperature between 10°C and 14°C. The cave entrance alone is 10 meters high, leading into chambers of enormous scale.
The cave contains a complex hydrology with numerous seeps, ten large pools, and several waterfalls. This high rate of water flow is responsible for carving the massive passages but has limited the growth of large stalactites and stalagmites. Instead, formations like flowstones, rimstone pools, and cave popcorn are more common. Within the main chambers, specific features have been named, including a formation resembling the Great Wall of China and another called the "Royal Throne". The cave hosts a unique ecosystem with 47 recorded species of wildlife, four of which are found nowhere else on Earth. One of these endemic species is the Hwanseon Blind Beetle (Kurasawatrechus latior).