The rules behind Bitcoin were put in place in 2009 by Satoshi Nakamoto, about whom very little is known. However, anyone can create new Bitcoins.

Fear of QE eating away at the real value of familiar currencies is one reason. Other buyers are simply speculating. Some do intend to use Bitcoin to buy things online.

Bitcoin's creators realise that, for anything to be valuable, it has to have some scarcity. So Bitcoin has been made difficult to create. You need powerful computers to do so, and the process is continuously being made harder. Some draw an analogy with gold. The metal is valuable only because it is rare – mining is costly and difficult. Creating new Bitcoin is called "mining" to stress the point. But even if someone – an expert programmer or hacker, say – found a way to make more Bitcoin easily, there is a limit of 21 million on the number that can exist. Each unit of the currency is tracked by the computer network to ensure that the limit cannot be breached.

How do you get hold of it?

It trades online at sites including eBay and various specialists. Prices can vary greatly.

Is it a good investment?

The price has gone through the roof recently. Some blame the crisis in Cyprus for undermining faith in normal cash deposits, although last week's huge expansion of QE in Japan could also have played a part.