Concrete Submarines (Terra Futura)

Concrete submarines were submarines  made of concrete. They could resist higher pressure than conventional submarines. No laughs, please.

Background
The submarine was one of the inventions of Cornelius Drebbel in 1620. It was made of wood and coated in leather. Oars were to used to propel the submarine. The first military submarine was the Turtle, built by David Bushnell in 1776. Its purpose was to destroy British warships in New York harbor. Although this failed, it was the beginning of the military submarine. Submarines were used in the Civil War. They were mechanically-powered and noisy. The most notable was the Hunley which, in 1864, sunk the Housatonic before sinking itself. With the advent of diesel-electric engines and torpedoes, submarines more common in navies. The most notable submarines during World War I and World War II were the German U-boats. During both those wars, U-boats sunk many ships like the Lusitania. During the Cold War, the United States built its own submarines. The USS Nautilus, commissioned in 1957 and named after the submarine from 20,000 Leagues Under the Sea by Jules Verne, was the first nuclear-powered submarine. The US Navy also built ballistic missile submarines, like the George Washington-class USS George Washington and the stealthy Ohio-class USS Pennsylvania, four of which would be converted to guided missile submarines. Most of these submarines had one thing in common. They were made of steel. That meant that the maximum depth of a submarine was 1,800 feet. Concrete submarines would change that.

Description
Tech Level: 11

This was no joke. Concrete submarines were a real concept. They were cheaper and easier to manufacture than conventional submarines. When the US started building concrete submarines, it saved them money. The use of  concrete meant that the submarine could go down deeper than a conventional steel submarine. Concrete submarines were also stealthy. It was difficult to detect them with sonar. Concrete submarines were most often deployed to the ocean floor. Because sonar could not detect unless they moved. The concrete submarine could then destroy an enemy ship from below. As concrete submarines had no buoyancy, they used electric turbine pumps to go up to the surface. The concrete mix had to be just because off the salt water and ocean pressure. Polymer coatings were also used to solve this problem. Concrete submarines were important during World War III. They helped the Allies win the war.