The price of the digital currency bitcoin hit a two-year high Monday, trading above $700 for the first time since 2014, after it jumped 21% over the weekend.

The surge came days before an unusual event called the “halving,” an adjustment to bitcoin’s protocol designed to control the creation of new coins.

The last time the currency traded at this level—as high as $719 Monday, according to data provider CoinDesk—was February 2014, when it was coming down from its December 2013 high of $1,147.

Bitcoin, launched in 2009, is a digital currency that runs on a decentralized network of computers and isn’t backed or controlled by any government. Trading volume is growing but remains thin relative to other capital markets, so a small amount of trading can trigger large swings in prices. About 400,000 bitcoins, worth about $279 million, traded hands from Sunday through Monday afternoon, according to Coincap, which follows virtual currencies.

Other factors could have contributed to the recent rise. Bitcoin is a popular speculative trade in China. And bitcoin, with gold-like qualities as a store of value, sometimes gets a boost when investors in broader markets start seeking havens.