Pchelen Kamak (Bee Stone) is a rock sanctuary. It consists of several rocks with hundreds of carved trapezoidal niches for religious purposes. Many of the rocks are deep inside the forest, and that makes them difficult to find. One of them is the engraved megalythic tomb Popmartinova Dupka (Popmartin's Hole). All these religious facilities were created by the ancient Thracians centuries before the new era.