Behold Ordinskaya, one of the World’s largest underwater caves

The Ordinskaya Cave is the longest underwater cave in Russia, only 20 meters at its deepest, but exceeding 3.5 kilometers in total length. The distance between the entrance lake and the furthest known underwater point equals 1 kilometer and 50 meters, with a diameter of 60 meters in the biggest underwater chamber, but with so clear water that two divers swimming at the opposite ends of the chamber can still see one another. Source: scaq.blogspot.com