The 2 major features of a submarine hull design that helps it withstand sea pressure at extreme depth is the material it's made of, and the design of the hull itself. Spheri
…cal or round-shaped objects can withstand more pressure than other designs. This is why the Teardrop and Cigar shaped hulls of today were designed. The hull's material is also key - the more it can compress and expand, the less vulnerable it is to pressure. This is a problem with old Russian boats like the Alfa - they're made of Titanium. While it's very strong, it's not elastic, and every time it goes to deep depths it becomes more brittle with age. Modern submarines, depending on the country, are made of either flexible steel, Titanium or GRP (Glass Reinforced Plastic). GRP boats are used by the Chinese, Titanium by the Russians. Most all other nations use flexible steel (the composition of which is classified) that compresses and expands with sea pressure as the boat descends or ascends through the depths. The steel used in pre-Seawolf class submarines was HY-80; The Seawolf and Virginia classes are using HY-100, which has a higher pressure rating. Submarine decks are not actually attached to the inner pressure hull - they "float" by being suspended from cables. This is how the decks keep from getting crushed as the boat's hull compresses at deep depths, as the decks don't actually touch the sides of the inner hull. (MORE) 
