An underground water system
The sea caves of the Golfo di Orosei are coastal features; they are exit points for a vast subterranean water system. The geology of the coastline is dominated by the Supramonte, a Mesozoic limestone and dolomite plateau. This type of rock is susceptible to karstification, a process where slightly acidic rainwater dissolves the rock, creating extensive underground networks of conduits and passages. Freshwater from rainfall on the Supramonte highlands travels for many kilometers through this network before emerging at the coast.
Caves like the Grotta del Bue Marino and Grotta del Fico are classified as anchialine caves. This means they have a subterranean connection to the sea and contain a mix of fresh and saltwater. In these caves, the less dense freshwater, flowing from the inland karst system, forms a distinct layer on top of the denser saltwater that penetrates from the sea through submerged tunnels. This interaction results from the coastal limestone geology. The entire cave system of Bue Marino is extensive, with over 70 kilometers of passages mapped beneath the Supramonte mountains.
The Halocline and its inhabitants
The meeting of two water bodies with different salinities creates a unique phenomenon called a halocline. In the Golfo di Orosei caves, this boundary is often clearly visible to divers as a oily-looking layer separating the cold, clear freshwater above from the warmer, saltier seawater below. In the Bue Marino cave, this boundary occurs at a depth of around 5 to 6 meters, with the freshwater maintaining a constant 15°C. In another cave in the gulf, the Grotta del Bel Torrente, a distinct halocline forms up to 350 meters from the entrance. The upper freshwater layer has a salinity of about 22 practical salinity units (PSU), while the underlying seawater measures around 33 PSU.
These stratified aquatic environments create specialized ecological niches. The caves host a range of stygobiontic fauna—creatures adapted to life in underground aquatic habitats. These often include specialized crustaceans like amphipods, isopods, and copepods. Many of these species are endemic, found only in this specific type of environment. They have adapted to the complete absence of light, often lacking pigmentation and eyes. The chemical gradients of the halocline directly influence the distribution and diversity of these organisms, which have evolved to thrive in very specific layers of the water column.