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.
At the start of 2017, the total value - or market cap - of all cryptocurrencies in existence was about $17.5 billion, with bitcoin making up almost 90 percent of that, according to industry data firm CoinMarketCap.
It is now around $120 billion - around the same value as Goldman and RBS together - and bitcoin makes up only 46 percent.
Bitcoin Cash, a clone of bitcoin that was split off from the original last week by a rival group of developers, was valued at more than $12 billion less than 24 hours after it had started trading.