German divers searching the Baltic Sea for discarded fishing nets have stumbled upon a rare Enigma cipher machine used by the Nazi military during World War II.

They think the historic find was thrown overboard from a scuttled submarine – and the divers initially thought they had found just a typewriter entangled in a net on the seabed of Gelting Bay.

But underwater archaeologist Florian Huber quickly realised the historical significance of the find.

“I’ve made many exciting and strange discoveries in the past 20 years. But I never dreamt that we would one day find one of the legendary Enigma machines,” Professor Huber said.

The Nazi military used the machines to send and receive secret messages during World War II. British cryptographers eventually cracked the code, helping the Allies gain an advantage in the naval struggle to control the Atlantic.

Christian Howe, Florian Huber and Uli Kunz with the rare Enigma machine. Photo: AAP

At Bletchley Park codebreaking centre, a British team led by Alan Turing is credited with unravelling the code, shortening the war and saving many thousands of lives. Dr Turing’s story was told in the 2014 movie The Imitation Game, starring Benedict Cumberbatch as the brilliant mathematician.

Shortly before Germany’s surrender in May 1945, the crews of about 50 submarines, or U-Boots, followed an order to scuttle their ships in Gelting Bay, near the Danish border, to avoid handing them to the Allies. Destroying encryption devices was part of the order.

“We suspect our Enigma went overboard in the course of this event,” said Professor Huber, who is from the Kiel company Submaris that leads underwater research missions.

Overall, the Germans sank more than 200 of their submarines in the North and Baltic Seas at the end of the war.

The Enigma device, which looks like a typewriter, consisted of a keyboard and wheels that scrambled messages. Although several hundred thousand machines were produced, only a few hundred are known to still exist. They sell at auction for tens of thousands of euros.

The find, made by divers working on behalf of the World Wildlife Fund to find abandoned fishing nets that endanger marine life, will be given to the archaeology museum in Schleswig.

-with AAP