(credit: Zach Copley) I used to be a bitcoin bull. As bitcoin's price soared from $13 to more than $1,000 in 2013, lots of people argued it was an unsustainable bubble. I argued the opposite: that bitcoin's price still had a lot of room to rise. And obviously, I turned out to be right, as bitcoin is now worth $17,000—17 times the cryptocurrency's previous peak in late 2013. Now we're in the midst of another big bitcoin bull market, and I'm much more worried that the market is getting into unsustainable territory.