(Tokyo)  --  The world's biggest exchange for bitcoins is filing for bankruptcy protection. 

That's according to a lawyer for Tokyo-based Mt. Gox, which lost an estimated $400 million worth of the virtual currency when it crashed earlier this week. 

Following that crash, the company's CEO said he was "working very hard" to fix the problems that shut down his site.  Bitcoin is an unregulated digital currency that is not backed by any government.