The "original" Karst (the limestone region of under-ground rivers, gorges and caves) which gave its name to all karst areas around the world, extends through a wide belt of south and south-west Slovenia, from Ljubljana all the way to the Italian border. Karst ia a type of terrain with a distinctive and uni-que assemblage of landforms and hydrology arising from a combination of high rock solubility and well developed secondary porosity, where special super-ficial and underground features (karst phenomena) developed and where underground (karst) water drainage is in control. These rocks are charasteristic Cretaceous and Tertiary carbonate deposits of shallow, warm-water carbonate shelf environments. A magnificent world under the Earth's surface - the stalagmite and stalactite rich karstic underground world is already very popular, thanks to the world-famous Postojna and Skocjan Caves. Other best-known show caves are the Pivka and Erna Caves, Planinska Cave, Vilenica and Krizna Cave.