The man had taken to the waters of Lake Waginger in the Bavarian Alps, on Friday to try and find his key armed “with a magnet on a home made fishing rod,” the police reported.
When the 28-year-old felt a weight on the end of his line, the man must have thought he was in luck. But he was in for a shock. Because when he pulled up his catch he found a rusty grenade stuck to his magnet.
“It was a live hand grenade from the Second World War,” a police spokesperson said.
But the man was quick to react.
“He responded very well and immediately dropped the hand grenade back into the water," the police spokesperson said.
Police munitions experts secured the explosive device shortly afterwards and defused it.
It is still very common 71 years after the end of the Second World War for unexploded munitions from the that era to be found in Germany.
Most often unexploded bombs are turned up during building work in the major cities, discoveries which sometimes lead to whole neighbourhoods being evacuated.
Small arms can present dangers of their own, though.
Earlier this month police had to be called in when a student at a school in north Germany brought a 17-centimetre shell into class which he had found in his family cellar.
Police said that if it had been dropped it could have exploded.
The beds of Bavarian lakes have also proven to be rich soil for unusual and valuable finds in recent times.
Last summer a 16-year-old girl found a gold bar worth €16,000 at the bottom of Königssee (King's Lake) while on holiday with her family. When no owner came forward, the teenager got to keep the treasure.