Where does the "air" go when a submarine submerges?

Question

Is this air compressed and held in large tanks or does the pressure within the whole ship rise slightly with no or little need of "pressure tanks"?

Answer 1

There are balast tanks that take it water and expel it over and over.  The pressure of the incoming water compresses the air, the sub can boost the pressure and expel the water.  I think they are close to neutral buoancy anyway so it does not take much to float/sink

Answer 2

There is an outer hull which is what the average person sees in the pictures of submarines.  Inside of that is a thick pressure hull.  Between the two are ballast tanks.  These tanks are open to the sea at their bottom (through large grates) and have vent valves at their top.  When submerging the ballast tanks are vented in order to flood them, neutralizing the buoyancy of the submarine and allowing it to submerge.

The air in the ballast tanks are vented to the earth's atmosphere.

Submarines now drive through their entire capabilities using the control planes of the submarine (be it rudder, stern planes, fairwater planes, or bow planes, depending on the class of submarine).  While there are tanks within the pressure hull of the boat that allow for adjustments in buoyancy and trim it is the control planes and momentum that do most of the work.

The submarine can drive to the surface and operate on the surface without blowing their ballast tanks.  These are pictures you see with the boat operating with its decks awash.  In order to send people topside and moor the boat the ballast tanks must be blown out.  Generally this is done a low pressure blower (with high volume).  Since the submarine is able to take in outside air by raising a mast it takes in outside air and pumps it into the ballast tanks, evacuating them of water, and bringing the submarine higher out of the water.

When submerged at depth and a submarine needs to surface there are compressed air banks air very high pressure air that are able to quickly expel the water from the ballast tanks and send the submarine to the surface, like the "USS Dallas" did in the movie "Hunt for Red October".

Answer 3

The air is exhausted from the tanks to the ocean/atmosphere. the sub carries more air compressed in tanks onboard. when they want to surface they put air back into the ballast tanks. They don't compress the air in the ballast tanks because they would have too much air to run through a compressor. The equipment to do that would be far to large and heavy to carry.

The dive planes point the sub up or down so as long as it is going forward they can make it rise or dive. The ballast adjustment is used to keep it stable and neutral where ever it is (or will be) in the water