It sounds like it might be something out of a James Bond movie, but Russian authorities say it's very real, indeed.
Russian TV has been showing off what looks like a rock, but is actually a sophisticated piece of spy equipment.
Russian State Television says that four members of the British Secret Service, posing as diplomats, put just such a stone in a Moscow park to electronically collect secrets from their Russian contacts.
The TV footage showed an X-ray of the rock, with a battery and memory board inside.
It's a 21st century version of the kind of dead drops that spies have used for centuries. Hollowed-out stones had been a favorite hiding place for secret messages.
The TV report said the British recruited at least one Russian agent, who was given a small handheld computer. Using the palmtop, the Russian could transmit data into the electronic memory hidden inside the rock. Later, members of the British Embassy in Moscow could pass by the rock and use their handheld computers to upload information from the stone.
The TV report showed surveillance pictures, supposedly taken last fall in Moscow, in which British diplomats passed the rock, and tried to retrieve information from it.
An unnamed agent of the Federal Security Service, the successor to the KGB, said that one Russian citizen had been recruited to spy for Britain and was given the equipment needed to send data to the disguised stone, but that he had been caught and arrested.
The British device would keep the Russian agent from having to meet with his British handlers to pass on information.
And it might have worked, except that the stone seemed to break down. The Russian TV report showed diplomats kicking the rock, and later taking it away for repairs.
The incident is giving the Kremlin a chance to attack two of its favorite targets at once: foreign spies and Russian human rights organizations. That's because one of the alleged spies working at the British Embassy also had responsibility for allocating British grants to Russian non-governmental organizations.
The TV footage showed internal embassy documents authorizing grants to organizations including the Moscow Helsinki Group, a human rights monitoring organization, and the Eurasia Foundation, which undertakes a broad range of activities aimed at strengthening Russian civil society.