Trim Tanks
Submarines use trim or ballast tanks to control both buoyancy and trim of the boat. Ballast tanks are simply containers that can store air or water in order to increase or decrease lift. If the nose is too heavy then air is injected into a forward trim tank to displace some of the water and the nose rises.

There are two types of trim tanks or ballast tanks; hard and soft. Hard tanks are fully enclosed tanks. Once the required amount of air is inside the tank, then the vents are closed and the volume of air remains the same even if the submarine ascends or descends.

Submarines use trim or ballast tanks to control both buoyancy and trim of the boat. Ballast tanks are simply containers that can store air or water in order to increase or decrease lift. If the nose is too heavy then air is injected into a forward trim tank to displace some of the water and the nose rises.

https://www.submarineboat.com/trim_tanks.htm

To control its buoyancy, the submarine has ballast tanks and auxiliary, or trim tanks, that can be alternately filled with water or air (see animation below). When the submarine is on the surface, the ballast tanks are filled with air and the submarine's overall density is less than that of the surrounding water. As the submarine dives, the ballast tanks are flooded with water and the air in the ballast tanks is vented from the submarine until its overall density is greater than the surrounding water and the submarine begins to sink (negative buoyancy).

https://science.howstuffworks.com/transport/engines-equipmen...

Para sumergirse o emerger, los submarinos usan los depósitos de proa y popa, llamados depósitos principales, que se abren y se llenan completamente de agua para sumergirse o se llenan de aire a presión para emerger. Durante la inmersión, los depósitos principales suelen permanecer inundados, lo que simplifica su diseño, por lo que en muchos submarinos estos depósitos son simplemente una sección del espacio entre los cascos.

https://es.wikipedia.org/wiki/Submarino

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Note added at 32 mins (2019-08-15 17:59:59 GMT)
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Submarines in a neutral buoyancy condition are not intrinsically trim-stable. To maintain desired trim, submarines use forward and aft trim tanks. Pumps can move water between the tanks, changing weight distribution and pointing the sub up or down. A similar system is sometimes used to maintain stability.

https://en.wikipedia.org/wiki/Submarine

tanques de proa

bow tanks

https://www.google.com/imgres?imgurl=https://i.pinimg.com/or...

https://www.pinterest.it/pin/502151427186796970/