A landscape shaped by ice
The limestone cliffs of the Tremiti Islands are riddled with caves, but the most geologically significant are hidden beneath the waves. The archipelago contains at least 40 known underwater karst caves, with openings located from just below the surface down to 50 meters. These are not sea caves carved by the action of waves, but rather terrestrial caves that formed in the open air. Their existence records the massive sea-level changes that occurred during Earth's recent ice ages.
During the Last Glacial Maximum, approximately 20,000 years ago, vast quantities of Earth's water were locked up in continental ice sheets. This caused global sea levels to drop dramatically. In the Adriatic Sea, the water level was between 90 and 130 meters lower than it is today. The Tremiti Islands, instead of an isolated archipelago, were hills on a vast, exposed coastal plain that connected modern-day Italy and Croatia. For thousands of years, rainwater, which is naturally slightly acidic, percolated through the islands' limestone bedrock, dissolving the calcite and slowly carving out a complex system of caves and passages. Inside these dry caverns, dripping water deposited minerals, building stalactites and stalagmites one microscopic layer at a time. The presence of these cave formations, known as speleothems, proves their subaerial origin. As the ice age ended and the glaciers melted, the sea level rose, inundating the coastal plain and flooding the caves, preserving them in their current underwater state.