Here's a impressive, and exciting submarine project. Well, it's exciting if you're interested in building pressure vessels with concrete. Now that's an obscure branch of nerd...
Back in 1996, Wilfried Ellmer built, tested and used a 20 ton concrete submarine on an Alpine lake. Residents of Lake Atter in Austria witnessed the launch of a pristine, streamlined, white vessel. The photo at right shows the boat at launching before the fitting out of sail, rudders and propellers.
Wilfried states that the 18cm (7 inches) thick hull gives an operational depth of 300m (~1000ft) and an estimated crush depth of about 900m. He used portland cement reinforced with 4mm and 6mm steel in a 10 cm mesh.
This is a large personal submarine, weighing in a 14 tons before ballasting. Check out the interior photo on the website which shows a roomy cabin. Unfortunately missing are any photos of the sub actually diving or even maneuvering on the surface under its own power.
