Tokyo bitcoin exchange Mt. Gox filed for bankruptcy protection Friday and also announced that 850,000 bitcoins worth roughly $473 million have gone missing. The exchange, which at one point was responsible for over 80 percent of trades for the virtual currency, also has outstanding debt of $63.6 million. Customers from places like the U.K. and Australia have flown all the way to Tokyo to try to recoup their money. "We are really sorry for causing trouble to all the people concerned," CEO Mark Karpeles said at a press conference on Thursday in Tokyo.