1680, Italian inventor Borrielli observed fish and found that most fish can adjust their body proportions by contracting or expanding the swim bladder, thus sinking or floating in the water. When the fish wants to float, the muscles relax, making the swim bladder bigger, the swim bladder filled with air, and the fish can float with greater buoyancy; If the muscles contract, the swim bladder becomes smaller, the buoyancy decreases and the fish sinks; A certain amount of gas in the swim bladder can also make the specific gravity of fish equal to that of water environment, so that the fish will stay there, neither rising nor falling.
According to the working principle of swim bladder, Bolie built a submarine with a diving bag made of leather, and controlled the ups and downs of the submarine by draining or injecting water from the leather bag. This is the earliest diving device.
There is a special water tank between the outer shell and the inner shell of a modern submarine, called ballast water tank, which is the ups and downs device of the submarine. In order to make the submarine float and sink quickly, modern submarines have also set up special diving cabins.
In recent years, submarines have developed rapidly, and the diving depth and speed have been greatly improved. American and French submersibles once set a world record of 1 1000 meters, which is conducive to a comprehensive understanding of the marine environment and seabed topography.