Deep beneath the waters of the Perm region lies the largest underwater gypsum crystal cave in the world. The Orda (or Ordinskaya) Cave in the shadow of the Ural mountains holds a vast mineral deposit of gypsum that forms an extensive underwater cave network–the biggest of its kind in the world.
Measuring 4,400 meters (14,435 ft) long and up to 43 meters (141 ft) below sea level, the Orda Cave is deep, dark, and frightening. With its countless winding passageways, caverns and galleries–many of which are still being found–the cave’s total length adds up to five kilometers. Adding to that thought its room temperature of less than zero degrees, the Orda Cave is somewhere you’d definitely not want to get lost
The cave’s rich gypsum content can be accounted for by its proximity to the Ural mountains, which is widely known in Russia for its mineral deposits. It’s also due to the mineral gypsum that the water in the caverns are greatly transparent. The waters are so clear that divers of the Cave can see over 50 yards ahead of them.
Source: Amusing Planet