Slovenia is famously known as a land of caves, with over 11,000 identified subterranean systems across its small territory. This abundance is due to the country's "Karst" topography—a landscape formed from soluble rocks like limestone that erode over millions of years to create intricate networks of caverns, sinkholes, and underground rivers. The term "Karst" actually originates from Slovenia's Kras region. While thousands exist, only about 20 are open for tourism, including the world-renowned Postojna Cave, which features its own internal railway, and the Škocjan Caves, a UNESCO World Heritage site home to one of the largest underground canyons on Earth. These caves provide unique habitats for rare species like the "human fish" or olm, an aquatic salamander that can live for a century.
That would be Slovenia.
While many countries have extensive cave systems, Slovenia is famously known as the “land of caves” and is often cited as having over 8,000 registered caves (some sources even say over 10,000). This is due to its unique karst landscape, which is where the geological term “karst” actually originated (from the Karst Plateau in southwestern Slovenia).
The most famous of these is Postojna Cave (Postojnska jama), a massive show cave with an underground train, and Škocjan Caves, a UNESCO World Heritage site known for its enormous underground canyon.
Other cave-rich countries include the United States (particularly states like Kentucky and Tennessee with Mammoth Cave, the world’s longest cave system) and China, but Slovenia’s high density of caves in a relatively small area is what makes its “over 8,000” figure so notable.