During late WW2 the Kriegsmarine toyed with the idea of a concrete U-boat but the idea was rejected not b/c it was impractical, but b/c there was a shortage of concrete and the war was lost. An Ex-Kriegsmarine officer in E Germany proposed the same thing to the Soviets as early as the late 1950's but technology did not permit the concept from becoming a realistic weapon. Later still, another German in the 1990s built a small concrete sub and this has led to a proposal for 20-200 ton concrete submarine yachts (prototyopes already built). Meanwhile, the Russians revived the concrete submarine idea with advances in technology. There subs would be of heavy construction with no need to draw water into tanks to submerge to unheard of depths that steel sub frames cannot match. US and Western subs can only dive down to max 2000 ft before the pressure becomes too great. The Russian C-subs could dive the entire way to the seabed floor out of range of counter-weapons, using external listening devices to pick up surface targets and then launching very high-speed torpedos to bridge the gap quickly, not allowing the target ship or enemy sub time to manoeuver before impact. The Russian C-subs are desingned with massive battery capacity to drive water turbines that have swiveling nozzles much like a Harrier jet and have mid-hull mounted lift wings to "fly" underwater at high speeds. Concrete ships are nothing new, many used in WW2 by the US as transport platforms... but so far only Russia is developing C-sub tech. The Germans have the tech to do it as well but are going with fuel-cell AIP attack subs that are largely high-tech automated craft with small crews of 27 or less and armed with advanced wire-guided STN-Atlas DM5 torpedos and anti-ship missiles.

http://orbitalvector.com/Aquatic/Concrete%20Submarine/concrete%20sub%20diagram.jpg