These are limestone caves, remains of the slate and limestone quarrying that took place for centuries along the foot of the mountain. Alum shale is one of the types of rock that make up the table mountains. The shale consists of alternating layers of sediments, some of which are richer in clay and others that are richer in limestone and organic material. They were formed in a sea around 500 million years ago. Here at Hunneberg, limestone can be found embedded in the alum shale. The limestone content in the shale occurs as nodules that are known as orsten. The alum shale contains oil, which meant that it could be used as fuel when the lime was burned, which also made it viable as a raw material.
The orsten was combined with the alum shale in lime kilns, and the shale served as the fuel. Traces of these kilns can still be seen close to the mining sites, together with reddish mounds of burned shale, known locally as rödfyr (“red beacon”). Lime has always been and still is a vital resource for people. It was originally used mainly for making lime mortar and as a soil improver, and more recently as one of the main ingredients in cement.
Diabase can also be seen in the roofs of the mining tunnels. This penetrated the various sedimentary layers of the table mountains as magma around 300 million years ago. The softer layers above the diabase have now eroded away, while the alum shale beneath the diabase survives.
Lime was produced on an industrial scale at Hunneberg from the late 18th century until the 1950s.
There are frequent rock falls inside the mine tunnels, so you definitely should not enter them. Alum shale also has a high content of uranium, which generates radon gas and poses a serious health risk!