Ever since Monday, bitcoin has been under pressure, and today the floor got pulled out from under it. At one point, Coindesk’s Bitcoin Price Index got as low as $524.33, down 7.7% from Wednesday’s close. It has since rebounded a bit and is now off “just” 4.1%, but that kind of whiplash is discomforting to anyone who’d bought into the thesis that the digital currency was starting to exhibit the stability that would bolster its appeal as a medium of exchange.