Researchers in China recently claimed that have taken a big step towards creating a 'supersonic' submarine. No one believes them.
Today we are all familiar with the Russian VA-111 'Shkval' supercavitating torpedo. The best that Soviet engineers could manage was pedestrian 230mph (200kts) for about 10 miles. So imagine sitting inside a one-man midget submarine averaging 300mph for thousands of miles, which you built in your shed?????
On August 11th 1966 Canadian resident and madcap inventor Josef Papp was rescued off the French coast. He was wearing an aircraft bone-dome and goggles, and told how he'd travelled from Canada in just 13 hours, underwater. Unfortunately his midget submarine had sunk without a trace.
It was of course a hoax, or a scam depending on how much faith you have in humanity. There is no way I believe he did it. His case was not helped by the fact that witnesses thought they'd seen him boarding an aircraft from Canada to France or that he had the plane tickets in his pocket. But he was a resilient character and maintained that he had built the world's fastest submarine. To put it into context, nuclear submarines top out at about 47mph (41kts) (-Alfa Class SSN) and most are much smaller. In fact anything over 20kts is very decent underwater. One way to go faster is to supercavitate, that is to form a layer of bubbles around you which reduce the resistance. That is how the Shkval and other fast underwater objects do it, and it is how Papp is claimed to have done it.
Papp was an interesting character. He'd escaped the communists in Hungary in the mid-1950s and wound up in Canada. The high-speed long ranged submarine was just his first major project and his later 'Papp engines' using 'Noble Gases' to generate motive power earned him millions of dollars and a luxury lifestyle, all for no provable achievement.