Just days after bitcoin started trading on two U.S. exchanges, the security of the cryptocurrency has been put into question as nearly $70 million worth of bitcoin has been hacked.

NiceHash, a company that mines bitcoins for its customers, is investigating a potential hack that saw hackers steal about 4,700 bitcoins from a wallet address used by NiceHash users, the Chicago Tribune reports. The company has announced that it will shut down operations for 24 hours while it conducts an investigation and confirms exactly how many bitcoins were stolen.

"The incident has been reported to the relevant authorities and law enforcement and we are cooperating with them as a matter of urgency," NiceHash said in a statement. The bitcoin mining company also advised its users to change their passwords.

Bitcoin has been growing in popularity and legitimacy, and, perhaps more importantly, in value. As of December 7, CoinDesk, a research company, valued bitcoin at $17,482 per unit. For reference, one bitcoin was worth less than $1,000 at the beginning of 2017. Bitcoin is now considered more valuable than McDonald's or Disney. 

However, this latest hack only helps to prove arguments against the cryptocurrency. Critics claim that it is useful only to those who use it for criminal or otherwise nefarious intent, as the anonymous currency makes it easier than ever to money launder or pay for illegal goods. This is of course not to mention that bitcoin is not, as this recent NiceHash hack reveals, entirely safe.

Supporters of bitcoin, however, claim that bitcoin can indeed be useful. As the Chicago Tribune notes, it can be a safer, more helpful method of payment in a crisis situation in which a national currency has collapsed, for example. Furthermore, there is a way to keep a running tally of transactions, making sure that even if bitcoin is stolen, it cannot be spent twice.