The Ljubljanica river basin is intertwined with the waters of Slovene natural sites, such as the intermittent Lake Cerknica, Rakov Škocjan, the famous Postojna Cave, numerous springs in Vrhnika, and of course, Planinsko Polje. On Planinsko Polje, the Ljubljanica is called the Unica. This is one of seven names of the karst sinking stream, which before the ice age, flowed on the surface, after which it gradually cut its way into the limestone. The water disappears in numerous sinkholes. If the water level is low, it sinks before it reaches the village of Laze. If the level is high, it spills out of its bed and across its meanders towards the northern edge of the polje, Pod Stenami, to the largest sinkholes. In the vicinity, there are two sinkholes called “Putikovi štirni” (Putick’s Wells), which are protected with nets to prevent debris brought by the water to accumulate and cause blockages. The wells were named after the Czech researcher Wilhelm Putick, head of irrigation works on Planinsko Polje. When the flow of the Unica in the south of the polje exceeds 60 m3/s, the sinkholes cannot absorb enough water causing the river to overflow its banks and flood the field. The field then becomes a lake in which only treetops are visible. Floods are most common in autumn and last, on average, a month and a half per year. The extent of floods is always very different. Water covers approximately 2 km2 during minor floods, and can cover up to 11 km2 of the region during major flooding. Extremely high flooding was recorded in 1820 and 1923 at which time Jakovški Hill became an island with water covering all the roads that led to the hill. The Unica is one of the most popular European rivers for grayling fishing. It also harbours other species of fish including pike, brown trout and carp.