Why are submarines always built with very thick and heavy metal?

Answer 1

The pressure hull of a submarine must withstand the pressure of water at depth. Hundreds of pounds of pressure per square inch translates into many tons of pressure around a pressure hull. If that hull doesn't have the strength to stand up to this, the pressure hull will implode. Steel is a "known quantity" as far as metals. We've used it for a long time, and we have a good handle on its characteristics. Steel alloys are extremely strong, fairly easy to fabricate, and won't double the national debt if we use this metal to make a submarine pressure hull.

Steel's weldability and its common availability make it a fine choice for submarine pressure hulls. It's really strong, too. That's a big plus. No one wants to be inside the pressure hull of a submarine when it fails.