Britain's top code-breakers say they are stumped by a secret code found on the leg of a dead pigeon.
The remains of the bird were found in a chimney in Surrey with a message from World War II attached.
Experts at the intelligence agency GCHQ have been struggling to decipher the message since they were provided with it a few weeks ago.
They say it may be impossible to decode it without more information - some of which could come from the public.
The message was discovered by David Martin when he was renovating the chimney of his house in Surrey.
Among the rubbish, he found parts of a dead pigeon including a leg. Attached to the leg was a red canister. Inside the canister was a thin piece of paper with the words "Pigeon Service" at the top and 27 handwritten blocks of code.
This was given to GCHQ at the start of the month.
We didn't really hold out any hopes we would be able to read the message because the sort of codes that were constructed to be used during operations were designed only to be able to be read by the senders and the recipients," said GCHQ historian Tony, who asked that only his first name be used.
He told the BBC: "Unless you get rather more idea than we have of who actually sent this message and who it was sent to we are not going to find out what the underlying code being used was."
'Dear Santa'The experts believe there are two ways the message might have been coded.
One is with a so-called one-time pad where a random "key" is applied to a message. If the key is truly random and known only to sender and recipient, the code can be unbreakable.
Another option is that this code was based on a specific - and now probably destroyed - code-book put together for a particular operation so the maximum amount of information about that operation could be relayed in the shortest message (this could be done in conjunction with a one-time pad).
There had been speculation that the message might have been sent by an agent of the Special Operations Executive and that it was heading for Bletchley Park. But these theories have largely been discounted. http://www.bbc.co.uk/news/uk-20456782