The largest Slovene karst polje is Cerkniško polje or Lake Cerknica. It is the largest European intermittent lake with a surface of 24 square kilometers.
During rainy periods in the autumn and spring, when numerous springs from the edge of the polje and underground streams collecting water under the surrounding plateaus, rash into the polje, and the sinkholes and siphons cannot handle the outflow, then the polje usually becomes Lake Cerknica.
In dry months the lake usually dries up.
Villages sit along the edge of the Cerkniško polje. The center of this part of Notranjska is Cerknica situated by the Cerkniščica stream under Slivnica ( 1114 m ) which has superb views. It was a parish seat as early as the 13th century.
Between Menišija and Slivnica stretches the Cerkniščica valley with its numerous tributaries and side valleys. In a small basin sits the village of Begunje through which passes a road to the Rakitna plateau. Over the last decades, Rakitna developed into a veritable holiday village.
The Lož Valley
To the northeast stretches the hilly world of the Bloke plateau ( Bloška Planota ) scattered with villages and settlements. The village of Bloke is famous because its inhabitants made their own skis centuries ago.
The road from Cerknica to the Loz Valley ( Loška Dolina ) passes the Križna cave ( Križna Jama ) which has 22 underground lakes!
Through the valley flows the Obrh River which springs at two different places – under the Racna gora with the highest peak Petelinjek ( 1212 m ) and Snežnik ( 1796 m ).
Lož, the first settlement in the valley was known already in the Middle Ages. It was granted a town charter in the 15th century. Atop the hill above Lož ( 698 m ) lie the ruins of Pusti Castle ( Pusti grad ). After WWII Lož lost its town status and the primacy in the valley was taken by Stari Trg pri Ložu.
Snežnik Castle sits near Kozarišče and is one of the most picturesque Slovene castles. It was first mentioned in writing 1268 as Sneperch. At that time it was in the possession of the Patriarchs of Aquileia who later presented it to the Lambergs. The building as it stands today dates from the 16th century and is built on a big rock surrounded by a natural water-filled gully. It houses a museum.
The Postojna basin is surrounded by rounded wooded karst hills except in the west where it is shut off by the steep Nanos plateau ( 1262 m ). The Pivka River flows into the valley from the south and is later joined by its tributary the Nanoščica, the latter springing from numerous sources under the Nanos. At Postojna, the Pivka enters the Postojna cave, one of the most beautiful European caves that can be visited by tourists. As early as 1819 the cave had already had 104 visitors, by 1901 their number exceeded 10.000 and until today over 25.000.000 people have visited the cave.
The underground current of the Pivka River can also be followed in the Black Cave ( Črna Jama ), which is also open to tourists, and the Pivka cave ( Pivka Jama ) as well as the Planina cave. All those caves belong to the Postojna cave system. The Karst Research Institute in the center of Postojna houses the Karst Museum and its collection. The first hotel in Postojna opened in 1874.
In the gaping mouth of a cavern under an overhanging rock on the slope of Podgorje, only 9 km from Postojna, at Predjama, is Predjama Castle of robber baron Erazem Predjamski, the subject of numerous legends. The original cave castle was first mentioned in the 13th century. In 1570 they built a new castle in front of it. Today this unique castle is an attractive tourist spot.
At the village of Zagon between Postojna and Predjama is the Betalov Spodmol cave where archeological finds proved that these parts were inhabited as early as the Stone Age.
Between Vremščica and the Brkini Hills, the Reka River has carved a narrow winding valley with hardly any flat surface in it. The valley starts to expand at Prem, a picturesque village with stone-built houses squeezed between the Romanesque Prem castle and the local church.
Ilirska Bistrica sits in the widest part of the valley between the Brkini Hills and the gently sloping wooded hillside of Snežnik ( 1796 m ).
The Reka River springs near Zabiče close to the Croatian border. Among the Brkini Hills, near Harije, not far from the road linking Ilirska Bistrica and the Materija Valley, lie two lakes: Mola, the lower lake with a surface of 74 hectares, and Klivnik – the higher lake with a surface of 36 hectares.
A road south from Ilirska Bistrica leads via Jelsane to the nearby international border crossing with Croatia.
Snežnik Castle was built on a rock in the middle of a lake. It is one of the best-preserved castles in Slovenia.
In a hidden valley, in the middle of a 123m high rocky cliff, is the never-taken Predjama Castle. This most picturesque Slovene castle is connected with the legend of the rebelling baron Erazem Predjamski. Under the castle is an entrance into the mysterious underground world.
The Pivka river simply disappears into a hill in which is the Postojna cave. Here we can enter an almost 20 km long system of shafts, caves, natural wonders, and eternal darkness.