1. Orda Cave: One of the world’s longest underwater caves. It is located underneath the western Ural Mountains in Russia. There is a local myth about a spirit that lives in the caves called the “Lady of Orda Cave”.
2. Yonaguni Monument: A rock formation near Japan. Marine geologists claim that this natural site has been modified or is a man-made artifact. It is a popular tourist attraction because of its large population of hammerhead sharks. Some claim it may be a key to a lost underwater civilization.
3. Shanghai Ocean Aquarium: It has a setup of 4 underwater tunnels that show 180 or 270 degree view of marine life. It is designed to give you a underwater experience without getting wet.
4. Underwater world in Singapore: It had more than 2,500 marine life from all over the world. The oceanarium was mostly underground. Although this was a popular tourist attraction, Underwater World closed down in 2016.
5. Gruner See: Located in Austria. Called Gruner (Green) Sea because of its emerald water. It is surrounded by mountains and forests. In spring, the basin fills up with fresh water from the snow that melts from the mountains. Divers are now banned from coming here because it will damage the sensitive environment.

https://todiscoverrussia.com/the-largest-gypsum-underwater-cave-in-the-world-orda-cave-in-russia/