Finsteres Loch, Natural cave in Rosenstein, Heubach, Germany.

Finsteres Loch is a cave in the Rosenstein mountain near Heubach with two natural entrances and an artificial opening called the window. The passage runs approximately 140 meters through the rock and connects several chambers together.

In 1919 the local researcher Franz Keller excavated in the entrance area and found thick burnt layers, showing that people stayed in the cave over long periods. Alongside medieval pottery sherds and many human and animal bones, he uncovered rings, knives and a Roman copper coin.

The cave's name, formerly written Finsterloch, refers to its dark interior. Excavations in 1919 by the local researcher Franz Keller uncovered thick burnt layers in the entrance area, along with medieval pottery shards, many human and animal bones, rings, knives and a Roman copper coin. The finds indicate people stayed in the cave over long periods.

The cave is reached only on foot, via partly unpaved forest paths on the wooded south side of the Rosenstein, about two kilometres east of Heubach. From early May to the end of September the south entrance stands open and no guide is needed; entry is free. There is no artificial lighting, the floor is unpaved and runs over scree and some steep sections, so bring a working lamp with spare batteries, sturdy shoes and robust clothing. Going in alone means no one will be there to help if you fall.

The side opening called the Window was cut by hand, as tool marks in the rock show; when and why it was made is unknown. The cave is the longest of more than 30 caves on the Rosenstein massif, and its three entrances are gated in winter for hibernating bats.