Russia's intelligence agency, the FSB, has accused British agents of storing and exchanging classified information using a fake rock on a Russian street.
A flap concealed the technology inside the alleged fake rock
It says the hi-tech stone is "absolutely new spy technology". The UK has not commented on the rock allegation.
The BBC News website's technology correspondent Mark Ward considers how such a rock might work, and the potential pitfalls of such technology.
Q: What kinds of devices could be used for this?
A: Any gadget that can swap data wirelessly would be able to work with the rock but the most likely candidates would be mobile phones and handheld computers - known as Personal Digital Assistants (PDAs). These are used very widely and it has become easy to use them to move data - be it a text message or an image or video clip - over short and long distances.
To avoid looking suspicious, those involved in the alleged data theft might have turned to a widely available gadget because the basic data transfer capabilities would have been built in and it would need minimal tweaking to get working. Many phone and PDA users regularly swap data, such as contact details or images, via Bluetooth short-range radio or via infra-red.
Q: What capacity do such devices have?
A: The latest devices can use wi-fi or even newer technologies to move data around faster. All can transfer many thousands of bytes of data per second. One byte is equal to one character and one million characters is roughly equivalent to the amount of data in a large paperback book.
Q: How would it work?
A: Without more details it is hard to be precise about how a rock and phone/PDA combination might work.
However, from what we know it appears that those who allegedly stole the confidential information walked close to the rock and then uploaded data to the device beneath it. Later, others came and downloaded the data and walked off with it.
It is most likely that radio of some kind was used to pass data back and forth. Infra-red demands a line of sight between the devices swapping data and would not penetrate the fake surface of the rock.
Many investigation firms use custom-built devices when trying to spot insurance fraud. Some have made "smart bricks" with cameras concealed in them which they toss into a suspect's garden. Typically these use short range radio to send images back to a van close by.
Q: What are the potential problems with this sort of thing?
A: The main problem is likely to be the battery life for the device beneath the rock that stores the data. Sending data via short-range radio is a notorious gobbler of battery power. If lots of data were being passed back and forth the useful life of the rock could be very short.
Q: Are there any other problems, more specific to this situation?
A: Yes. One is the speed at which data can be uploaded or downloaded. While standing in the street tapping on a phone or PDA does not raise much comment any more, it would if you had to stay in the same place for hours. But using faster speeds tends to be even more battery hungry.
Another problem is ensuring that only the right people upload and download data. Unprotected wi-fi networks are notoriously easy to find and log on to. It might prove hard to find ways to continually update the alleged spies on regularly changing passwords, encryption keys and the like.
The final problem is the clumsiness of this method for passing data. If a PDA has a wireless link there are likely to be lots of public places that would allow that data be sent more anonymously. For instance, many tech-savvy criminal gangs use hijacked servers to store stolen data.