An underground city. Although it isn't super big at the surface of the ocean floor, there are massive tunnels leading to buildings and other areas underground. Get prepared to eating Calamari here. It is actually the largest city on the world. There is also a submarine tour service, where you go on a tour through the city in a blue-whale sized submarine that is actually based on those behemoths. You can be driven (piloted?) to an underwater, dormant volcano a few miles away. Above this volcano is a massive, floating metal and glass ball half a kilometer in diameter. In this is the gym.