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 the extreme 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.
Hulls that don't flex with sea pressure, while being able to have deeper operational limits, don't have the longevity that flexible hulls do. This is because over time, repeated pressure makes the hull brittle - it is akin to repeated pressure on an eggshell. While very strong, over time it will eventually crack.
Today's attack and ballistic submarines are made from special steel and/or titanium that are needed to withstand the pressures of the 1,000' depths at which they operate. US submarines use a hi-pressure, flexible steel known as HY-100, while the Russian boats are made from very expensive titanium.
Most submarines were made of steel.
Both sides, the Allies and the Axis, had submarines when WW2 started.
Steel or titanium
All submarines were built from steel. Except Bushnell's Turtle in 1776, that was made of wood & leather.
Submarines are made in many places all over the world
the military and navy use it for war and combat. Many submarines are made for launching missels and taking down ships and other subs.
Submarines are used both for military and rersearch purposes.
Germany made the first submarines in WW1. They were called U-Boats, short for "unterseeboat" Most modern submarines are descended from The Holland, christened in 1897, invented by John Holland.
About 2,100 by many different countries.
Germany made its own submarines and had been doing that since the days of WW 1, long before Hitler.
Most submarines are made mainly of steel with bits of many other materials.
There are quite a few devices that have made sea exploration as you know it possible. Submarines are one of these devices.