That little brown bottle that you have in your bathroom cabinet, the one that bubbles on cuts, once held the key to a revolutionary new form of submarine. This sub, if it had been mass-produced, might have made Germany a lot harder to beat in World War II.
Submarines in the pre-WWII era ran on regular fuel. They managed to go about eleven knots on the surface of the water, and seven underneath it. Seven knots is slower than a boat, which made it hard to catch the boats they were attempting to sink, and relatively easy to be caught by them. There was also the problem of having a boat stumble upon a sub when it surfaced. Subs had to surface a lot. Fuel combustion takes oxygen — something which is at a premium in a submarine. Underwater, to keep the entire crew from suffocating, the sub ran on relatively weak batteries, which needed to be recharged on the surface often.
Most people focused on the problem of supplying oxygen to fuel underwater. Hellmuth Walter had another idea. He knew there were a lot of molecules that carried their oxygen with them. Hydrogen peroxide was one of those molecules. Hydrogen peroxide is H2O2. With a catalyst, it splits into water and oxygen gas. The split comes with a lot of heat, so much that the water becomes steam and can power a turbine. The oxygen that is released can be used to burn any auxiliary fuel.
Walter attempted to get the company he worked for to look into hydrogen peroxide research, but for years was met with indifference. When he finally did get the attention of the German government, his initial tests were so successful that he could start his own company. In 1940, he made a small model of a submarine - which held just a few people - with a stable form of hydrogen peroxide called perhydrol as fuel. It managed to do 23 knots submerged.
Unfortunately for Walter and his dreams - but fortunately for everyone else - the war was in full swing and there wasn't any material, manpower, or space leftover to change the assembly lines and make these new boats. Training people to maintain and operate them was another burden. Although three were made before the end of the war, none ever saw combat. None even managed to make it past the crew training. Those who do backseat warring believe that if the Walter Boats were launched, they might have prolonged World War II. By the end of the war and the beginning of the nuclear age, the boats were left behind. They'd missed their era of relevance by a few years.