Mt. Gox was one of the earliest and largest Bitcoin exchanges, processing hundreds of millions of dollars worth of Bitcoin trades every day - right up until it closed back in 2014. Mt Gox announced that they had been hacked, and that 850,000 bitcoins had been stolen. But because all Bitcoin transactions are recorded in the public blockchain, researchers were able to track all of the coins as they moved around the network, and eventually even to figure out who was likely behind the hack.
Everytime you send Bitcoin, information is recorded on the blockchain about your address, the address you sent to, and how much bitcoin was sent. And while no one knows who owns those addresses, anyone can see how every bitcoin flows through the network, stey-by-step. That’s why Bitcoin is anonymous (no one know who is moving money), but not private (because everyone can see the money moving). In October of 2016, Zcash was launched with the goal of making a making a cryptocurrency that was both anonymous and private.