Divers on a WWF expedition to search for ghost nets in the Baltic Sea turned up a more unusual discovery last month. What was initially mistaken for an old typewriter turned out to be an Enigma code machine used by the Nazis in WWII.

The infamous encryption machines were used by the Nazis to encode messages sent via Morse code during the war, scuppering the Allied Forces’ attempts to track German military movements. That is until a British team of mathematicians and scientists – led by Alan Turing, and building on breakthroughs by Polish codebreakers – broke the encryption in 1941; something historians credit with shortening the war.

The German divers were searching for abandoned fishing nets, known as “ghost nets”, which can be deadly to marine life, in the Bay of Gelting in northern Germany when their sonar picked up what they thought was a typewriter caught in a net. The letters were still legible.

"A colleague swam up and said: there's a net there with an old typewriter in it," Florian Huber, the lead diver, told the DPA news agency.

They quickly realized it was something of much more historical importance. Huber suspects it was lost shortly before Germany surrendered in May 1945, as submarines in Geltinger Bay were ordered to be scuttled by Nazi leaders to prevent their capture by the winning Allies.

Over 50 U-boats were deliberately sunk in the bay on May 4. “We suspect that our ENIGMA went overboard in the course of this event," Huber said. It is thought the machine, which is a three-rotor Enigma, was thrown overboard from a ship rather than a submarine, as Nazi U-boats used the more high-tech four-rotor machines. 

The divers handed the find into the archaeological office of the German state of Schleswig-Holstein, where it will now be restored by experts at the state's archaeology museum. Having spent 70 years at the bottom of the sea, it will need to go through a thorough desalination process to prevent further corrosion. 

The device essentially consisted of a keyboard, a set of three or four rollers, and an illuminated field of letters. The operator would type the message in plaintext and then the converted ciphertext would be reflected in the lights above the keys. Enter the first letter of your message and the corresponding letter would light up. Cipher keys were changed regularly using a series of device settings that created lists used by the sender and receiver. It was the insistence of signing off messages HH (Heil Hitler) that allowed the code breakers to find the weak spot in the encryption and break the code.

Germany, which doesn't shy away from teaching the lessons learned from its past, will then put the Enigma machine on display. "Due to its historical significance, it can even be viewed as a national cultural asset," Dr Ulf Ickerodt, director of the state archaeological office of the State of Schleswig-Holstein, said. 

Accidental finds like this are rare and come from a period in Germany's history that is increasingly in danger of being forgotten, Dr Ickerodt said. "Despite all the fascination with the history of technology, which is not just about cryptography but also about the early phase of modern information technology, they are a welcome opportunity to deal with this phase of our history."