Why are pressurized tanks required for deep scuba dives?

Because pressure increases the further down you go. A unpressurized tank would collapse under this weight. Actually, the pressure in the tank is not to prevent it from crushing, but to allow it the diver to breathe. The greater the pressure in the tank, the greater the amount of breathing air you have available. As ambient pressure increases (ie by descending), your lungs require more air to fill them, as you are then breathing compressed air into your lungs. If you tried to breathe atmospheric pressure at depth, you could not fill your lungs. The deeper you go, the harder it is to breathe... Try breathing through a 6ft snorkel to the surface, and see how difficult it is to inhale. Therefore, the tank is pressurised, 1/ to allow you to breathe effortlessly at depth, 2/ to give you more time at depth, and 3/ to keep water out of the tank.