Sea cave with 200m underground lake. Salinity profiling shows halocline at 1m depth where fresh groundwater floats on seawater, creating two-layer ecology in a single pool.
Holger Uwe Schmitt, CC BY-SA 4.0
A Lake of Two Layers
Inside the limestone cliffs of Capo Caccia is a large, anchialine cave system that contains a 120-meter-long salt lake, Lago Lamarmora. This body of water is one of the largest underground salt lakes in Europe. The lake's most scientifically interesting feature is not its size, but its structure. It consists of two distinct layers of water that do not mix. A layer of cooler, fresh or brackish groundwater, approximately one meter deep, floats on top of a more extensive, warmer layer of seawater.
This layering occurs because of the density difference between the fresh water percolating down through the porous limestone and the denser salt water that enters the cave through submerged passages from the Mediterranean Sea. The sharp boundary between these two layers is a halocline, a zone where salinity changes rapidly over a small change in depth. This stratification creates two separate aquatic environments within the same lake. Each layer supports a different community of organisms adapted to its specific salinity, temperature, and chemical composition.
Geology of the Grotto
Neptune's Grotto is a karst cave, formed over millions of years by the dissolution of Mesozoic limestone. The entire cave system is estimated to be around four kilometers long, though only a portion is accessible to the public. The formation began roughly two million years ago as rainwater, made slightly acidic by atmospheric carbon dioxide, seeped through cracks in the rock, slowly dissolving the calcium carbonate and carving out the vast chambers and passages seen today.
The cave is famous for its varied speleothems—mineral deposits formed from dripping water. These include stalactites, which hang from the ceiling, and stalagmites, which grow from the floor. A constant dripping over a century is required to form just one centimeter of a stalactite. Inside, several large chambers contain notable formations. The Smith Cavern holds the "Great Organ," the largest column in the cave, which is 12 meters high and 6 meters wide. Another prominent feature, a two-meter-tall stalagmite known as the "Acquasantiera" (Holy Water Font), collects fresh water in small pools on its surface, providing a water source for birds.
💡Fun Facts
A local fisherman discovered the cave in the 18th century.
The land entrance is reached by the "Escala del Cabirol" or "Goat's Steps," a 654-step staircase built in 1954.
The grotto was a filming location for the 1979 science-fiction movie "Island of the Fishmen".
The cave was once a habitat for the now locally extinct Mediterranean monk seal.
Open daily, but hours vary by season. Summer hours are typically 9:00 AM to 7:00 PM; winter hours are shorter. Access is always dependent on calm sea and weather conditions.
Admission
An admission fee is required to enter the cave. Boat fare is a separate cost.
Accessibility
The grotto can be reached in two ways. One is by a boat from the port of Alghero. The other is by land, which requires descending and ascending the 654-step Escala del Cabirol carved into the cliff face. The boat is the only way to avoid the stairs.
