Not limited to a specific municipality, the Karst (sometimes referred to as 'Kras') region is the land situated between Trieste Gulf and the Vipava Valley. The limestone plateau is so picturesque, and is famous for lending its name to a specific topographical phenomenon wherein fusible stones are affected and shaped by water. The lakes, springs, swallow holes, caves, and potholes that qualify as Karst make up a large portion of Slovenia.
As such, Karst is most famous for its incredible caves. These include Vilenica, Lipica, Divača, Kacna Jama, Postojna, and the Škocjan Caves, which is a UNESCO World Heritage site. These are the main tourist attractions, and are certainly not to missed during a visit to Slovenia.
There are around 100 municipalities within the region, the center being Sežana. The main rural centers include Divača, Dutovlje, and Komen.
Cerknica Lake appears between October and June, and provides a wonderful atmosphere for fishing and bird-watching.
Because the region is so widespread, the most effective way to see it is by car. If you're flying into Ljubljana, it is possible to rent a car, take a taxi, or get a bus into any major city in the region.
The geological haven requires a good map and a good idea of how you would like to spend your time, as the lack of centralization can be confusing.