The Cave of Koutalas on Serifos: The cave of Koutalas, also known as the cave of Cyclops, is located to the west of Koutalas beach, on the southwestern part of Serifos island. The cave was unknown for centuries until it was accidentally discovered in 1893 by miners who were looking for iron and copper. The entrance of the cave had been covered by a large stone surrounded by mud and its side walls were almost collapsed, which is why it remained unknown for so many centuries.
In the interior of this limestone cave, there are many stalagmites and stalactites with strange shapes. Speleologists believe that the cave of Koutalas was formed gradually during the tertiary geologic time periods about 2 million years ago. Some of the relics found inside the cave show evidence of its usage as a worship place in the prehistoric times. The Cave of Koutalas can't be visited today because of the fear to collapse.