Planinska jama (The Planina cave), one of the longest Slovenian
water caves is a huge tunnel which is the subterranean bed of the Unica
River. Five hundred meters from the entrance into the cave is a confluence
of two underground rivers: Pivka River, which flows from Postojna polje
through Postojna cave, and Rak River, which flows from Rakov Skocjan. This
is the largest confluence of subterranean rivers in Europe. Entrance
into the cave is on the southwestern part of Planina polje in the pocket valley below the 65 high rocks. The length of the cave is 6.656 m.