The Classical Karst (Matični Kras) is a plateau of carbonate rocks, elongated in the north-west south-east direction, located in an area of almost 700 sq km, divided between Slovenia and Italy. Borders: to the north west the floods of the Isonzo river (Soča) with the Redipuglia relief as the westernmost point, up to the Timavo resurgences; from south-west to south-east the limit is naturally represented by the north-eastern Adriatic coast of the Gulf of Trieste and the lithological contact with the Flysch to the south-west, continuing in the same direction, up to Val Rosandra (Glinščica); from here the perimeter is less clear, continuing north-east to and beyond the area of the Škocjan Caves, where the Reka River (upper Timavo) sinks, leaving behind (to the south-east) its flyschoid basin; to the north The limit goes from the southern slopes of Monte Auremiano / Vremščica continuing in a north-westerly direction towards the Raša stream, the Branica river and the contact between the Flysch of the Vipava Valley (Vipava) and the cabonatic rocks until it rejoins at the confluence between the Vipava and Isonzo rivers.
The calcareous substrates are very permeable, causing widespread aridity that can be locally exacerbated by the heat-reflecting action of the bare rocks. The high permeability of this substrate is due to the fracturing of the limestone rocks which allowed the sinking of the water networks, now totally underground, resulting in a hydrography that develops at a depth of 200-500 m (Poldini, 1972). The hypogeal karst also manifests itself with cavities that generally follow the lay of the layers and with pits that form in correspondence with sub-vertical fractures. The karst lakes of Doberdò and Pietrarossa and the Rosandra stream are almost the only example of surface waters.
The karst lakes of Doberdò and Pietrarossa are a rare example of karst lakes without surface rivers that act as tributaries and emissaries. Another example of a sui generis lake is that of Cerknica, in Inner Carniola. The waters flow into them through resurgences and underground rivers in addition to the pluvial supply; the outflow is instead guaranteed by underground cavities and evaporation. The hydrology of the Doberdò and Pietrarossa lakes has been the subject of various studies and researches.