Bitcoin and other "cryptocurrencies" are big money, virtually as big as Goldman Sachs and Royal Bank of Scotland combined.
The price of a single bitcoin hit an all-time high of above $3,500 this week, dragging up the value of hundreds of newer, smaller digital rivals in its wake. Now some investors fear a giant crypto-bubble may be about to burst.
It has been a year of unprecedented growth for the largely unregulated market, with dozens of new currencies appearing every month in "Initial Coin Offerings" or ICOs. They have achieved value almost instantly, drawing in those who are eager to get in and make a quick buck.