A lot of people were surprised when Bitcoin’s price surged to $10,000 last week—a more than tenfold increase since the start of the year. But this week Bitcoin’s rally has actually accelerated. Bitcoin’s price passed $12,000 yesterday. This morning, it surged past $15,000. As I write this, one bitcoin is worth about $15,200.

The astonishing increase means that all bitcoins in circulation are now worth around $250 billion. It’s not clear why Bitcoin’s price is rising so quickly.

Over the last year, the currency’s popularity has been buoyed by a larger phenomenon of initial coin offerings—sales of cryptocurrencies that compete with bitcoins. For many people, the easiest way to acquire these alternative cryptocurrencies is to first buy some bitcoins from a bitcoin exchange, then trade those bitcoins for another currency.