Tucked beneath the western Ural mountains is Orda Cave, one of the most dangerous caves in the world to explore. These caves near the Russian city of Perm are also one of the world’s longest underwater caves, stretching for more than three miles. A major portion of the cave is submerged in crystal-clear water due to the rich gypsum rock of the mountains. Tucked beneath the western Ural mountains is Orda Cave, one of the most dangerous caves in the world to explore. These caves near the Russian city of Perm are also one of the world’s longest underwater caves, stretching for more than three miles. A major portion of the cave is submerged in crystal-clear water due to the rich gypsum rock of the mountains.