Dobrostanski biser, Show cave in the Rhodope Mountains, Bulgaria

Dobrostanski biser is a pothole cave in the Rhodope Mountains, within the Chervenata Stena Reserve, about 5 km from Dobrostan village and a 20-minute walk from Martsiganitsa hut. Its entrance shaft drops 15 metres. The chamber is filled mainly with stalagmites and thick stalactones, some covered with a greenish layer of algae and mosses, and large gravitational blocks lie in the centre of its south-western part.

The cave was first explored in 1963 and opened to tourists in 1990. It was later closed, and thieves stole the cave pearls and destroyed the cave pools. It was renovated and reopened to visitors in 2018, with visits on Fridays, Saturdays and Sundays.

The cave is also known as Ahmetyova Dupka and Protkletata. The first name recalls a Turkish tax collector, Ahmed Aga, linked to the site in local memory; the second means "the cursed one". Visitors enter through a 15-metre shaft into a hall where greenish algae and mosses coat some of the thick stalactone columns.

The cave is open for visitors from Friday to Sunday and typically requires guided tours. Comfortable shoes are helpful as the floor can be uneven, and a flashlight is useful for navigating the passages.

The cave holds a surprising collection of natural rock formations that spark the imagination. Visitors report spotting shapes resembling a unicorn, a turtle, a flying saucer, and even a castle.