The so-called “Stone Mushrooms” are a remarkable natural phenomenon in the Eastern Rhodopes, located at the road from Kardzhali to Haskovo, north of the village of Beli plast. The rock formations are 2,5 to 3 m. high  and occupy an area of 3 ha. They actually represent mushroom-shaped rhyolite volcanic tuffs and their coloring is a result of diverse mineral composition. They’re referred to as “mushrooms” because of their shape- they resemble natural mushrooms, as the trunks are pink and the “hats” - greenish.

The tuffs are a result of underwater volcanic activity, dating back to the Paleocene, when the area was at the bottom of a prehistoric sea. After the water withdrawal and the consistent seabed elevation, erosion started affecting them slowly. The soft pink layer easily succumbed to the action of the natural forces. On the other hand, the volcanic glass in the upper greenish layer was solid and has preserved to a greater extent. Thus, the influence of nature has created a wonderful combination of unique shapes and colors.

The area was declared a natural landmark and is inhabited by some rare bird species such as the Egyptian vulture, the Short-toed eagle and others. These rock formations, as well as others in the area, have a very intriguing story. The legend says that after a horde of invaders scared the four daughters of Radul, the girls killed the leader’s horse. To have his revenge, he started killing them. One by one, the girls turned into stone statues. Eventually, while trying to escape, the horde leader tuned into a black rock himself. To this day, people still call the lonely crag, near the Mushrooms, Karatepe ( tur. "Black hill”).