Mt. Gox was once Bitcoin's biggest exchange. After first slipping in the ranks to several newcomers, the exchange shut down its website and announced that hundreds of millions of dollars worth of customers' bitcoins had been lost to a series of hackings that had gone unnoticed over several years. It's since filed for bankruptcy protection and began to search, with some success, for the missing currency. Customers are of course irate, and there's no indication that they'll be able to get back the money that they had stored with Mt. Gox.