Mt. Gox was one of the earliest and most public downfalls of the Bitcoin era. In early 2014, Mt. Gox stood atop the field of Bitcoin exchanges as the largest, until it declared bankruptcy following the disappearance of almost $500 million Bitcoins and cash from its coffers.
The exchange lost 850,000 Bitcoins with a value of about $450 million in February of 2014, alongside an additional $27 million in cash. Some 200,000 of those Bitcoins were eventually found, leaving 650,000 still missing. In the time since, many analysts, former Mt. Gox investors, and others have speculated as to where the missing currency is. This is particularly important as Bitcoin's price has soared in recent months: the missing Bitcoins could be worth as much as $2 billion at this point.
Read more here: http://www.investopedia.com/news/wheres-missing-mt-gox-bitcoin-now-worth-2-billion/