Gradišnica is a 201-metre abyss on the north-western slope of Gradišče. The eastern part of the abyss measures 25 x 35 metres and is 65 metres deep. Under the entrance, the steep Kraus’s Corridor descends to a depth of about 130 metres to where Hauer’s and Putick’s Halls lie.
Several tunnels ending in siphons lead from the halls. The water level in the cave fluctuates by about 60 metres. High water from the Ljubljanica occasionally appears in the cave and runs underground from the sinkholes at the edge of Planinsko Polje to the springs in Vrhnika. J. V. Valvasor discovered Gradišnica and named it the Vražja Cave. Information and visits are available by prior arrangement from: (Logatec Caving Club).