At the exit from the canyon, on the right, at the 104th kilometer of the Una stream, there is a very generous and mysterious Black Lake, which is actually a karst spring whose origin is little known. The source appears in the Una canyon, under the bridge over which the road from Bihać leads to Bosanska Krupa, 5 km before the town itself. It is at the same level as the river, into which it immediately flows, with a water flow that is only apparently zero. The remains of the former wooden bridge form the border of the water surface. The lake is very clear, but due to its depth and eternal shadow, it appears dark, which is probably why it is called Black.

At a depth of 10 meters, on the bottom, there is a small passage through boulders, behind which is a spacious entrance to the cave measuring 4×2 meters, where a large gallery begins, the diameter of which is about 3 m high and 5-8 m wide, which is sometimes over 10 meters, so despite good visibility, the walls of the gallery cannot be seen. In the surface part, the temperature of the Black Lake is 11°, and in the depth it is 9°, which means that the water of this source comes from great depths.

The 140 m long gallery descends at an angle of about 45º, and a depth of 65 m has been determined, which is not final. The main direction in which the gallery extends is towards the east, in the direction of Grmeč, whose entire surface lies on distinctly karst soil, and from whose northwestern slopes this spring is probably fed. Various specimens of speleo-fauna were observed in this source, which is characteristic for the entire Grmečki basin, and in addition to monolistry, niphargus includes specimens of the human fish Proteus anguinus.

According to its characteristics, this source is very interesting, because hydrologically it does not function according to the usual pattern like other sources in the wider environment, because it is not a rare case that all sources, including Krušnica, cloud their water after heavy rains, or due to the spring melting of snow from Grmeč, but this does not apply to the Black Lake, which in such circumstances continues to provide clear water without a significant increase in the level. This only confirms that the Black Lake comes from a great depth and a very long underground karst system, in which it is purified before enriching Una with its water. Black Lake is simply a natural wonder of the Bosnian Krajina that will enchant you with its beauty and mystical atmosphere! The lake is a true paradise for lovers of nature, adventure and peace. Its dark and mysterious water creates an enchanting contrast with the surrounding greenery and rocks.