How does a submarine submerge without sinking

 
 
In lakes, rivers and seas there are microorganisms that live freely in the water. They form the so-called plankton. Some of these organisms can swim with hairs or the like, but most of them lack such structures.
 

Yet some can control their position in the water:
Similar to the way fish have a swim bladder, they have gas vacuoles. These are small cell structures that are filled with gas in order to achieve greater buoyancy.
For example, the plankton can avoid predators or go to a height where there is just enough light or nutrients.

Eudorina

 
 
 

The trick with the submarine

 

Man has recognized this principle and uses it in the submarines. They also use the trick of changing their own density to control the height in the water.

 

Submarines have large ballast tanks that are filled with air when floating on the surface.
This makes them lighter overall than water. To submerge, the tanks are filled with water. The submarine gets heavier than water - it sinks.
To get back to the surface, the helmsman fills the ballast tanks with air again. This comes from large compressed air cylinders that are carried for this purpose. The air displaces the water in the tanks, the density of the entire boat becomes less than that of the water, and the boat emerges.

Submarine of the German Navy © dpa