And if so ,what can sink a Submarine ?

-If by "Sink" you mean can they go beneath the surface, the answer is yes. If they couldn't, they'd be damned poor submarines. A more serious answer would also be yes. "Air" has little or nothing to do with staying afloat. The answer is displacement, as discovered by Archimedes more than two millenia ago. If a vessel displaces a greater weight of water than it weighs, it'll float. One way to accomplish this is to fill the interior with low density materials such as gases. However, if a submarine or other vessel were utterly evacuated (pull a vaccuum on its interior) it would float even better than if it were "filled" with air.

-A Sub usuallh has ballast takns in an outer hull, and an inner hull which stays full of air to sustain the crew. Nuclear Subs, like the Los Angeles class, usually have scrubbers to help keep the air breathable for a long time underwater.

Water being allowed in to ballast tanks between the hulls, while planes on the front (bow) and back (stern) are angled down - these planes are like rudders mounted sideways, and can help the boat go up or down.

To make the boat come up to the surface, the planes are pointed up, and high pressure air (like that one would have in an air pump or a CO2 cartridge) is used to blow the water out. With the water out and the planes up, up comes the boat.