The price of the world's best known digital currency briefly crossed the $5,000 mark on a major index for the first time on Friday evening ET, before retreating about 5% in subsequent hours. The idea of Bitcoin breaking the symbolic milestone of $5,000 would have been unthinkable to most people at the start 2017, when the price topped $1,000 for the first time. If you're keeping track, the digital currency is up 500% this year, and nearly 2200% since mid-2015, when it was in the doldrums at around $220. There appears to be no single reason for the recent run-up. Instead, it can likely be explained by the same factors driving this year's cryptocurrency bull run: Publicity-driven speculation; New financial products creating unprecedented liquidity; Trading surges in Asian markets; Institutional investors treating digital currency as a permanent new asset class.
Meanwhile, the $5,000 milestone is likely to trigger a new round of chatter that Bitcoin and other cryptocurrencies are in a bubble and vulnerable to a major price collapse. Bitcoin has experienced a series of spectacular crashes in the past (most recently in 2014 when it dropped around 75%) but has always recovered. Get Data Sheet, Fortune's technology newsletter. Finally, it should be noted that Bitcoin crossed $5,000 on an index used by the trade publication Coindesk, but not by other major indexes. This is significant because Coindesk's BPI index includes prices from several Asian exchanges, where prices are typically higher than US or European one.
A more conservative price estimate can be found on an index created by the Winkelvoss twins, are among the world's biggest holders of Bitcoin. Known as the Winkdex, the index only draws data from U.S.-dollar denominated exchanges. As you can see, the index's calculation shows how Bitcoin prices approached the $5,000 mark, but did not break it: Bitcoin is not the only cryptocurrency to achieve new highs this week. Ethereum nearly crossed the $400 mark for the first time while another smaller rival, Litecoin, briefly broke through the $90 mark. As of Saturday late morning ET, Bitcoin was trading between $4,500 and $4,600.