A massive database containing 772,904,991 unique email addresses and more than 21 million unique passwords were recently posted to an online hacking forum.

The hack was first reported by Troy Hunt of the hack security site “Have I Been Pwned”, which lets users check whether if email addresses and passwords have been compromised and which sites the information was leaked from.

The data collection as a whole, called “Collection #1,” doesn’t appear to originate from a certain source, but is rather a collection of 2,000 leaked databases that include passwords that have been cracked.

The data in Collection #1 wasn’t put up for sale, like many data leaks are.

It was first hosted on popular cloud hosting site Mega before being taken down, then posted on a public hacking site.

“Collection #1” is among the largest data breaches in history, second only to Yahoo’s hack that affected as many as 3 billion users.

To check whether or not your data was affected, individuals can head over to HaveIBeenPwned.com and enter their email address.

If data was affected, the site will indicate that the data was included in the “Collection #1” leak.

Afterwards, one can head over to the “Passwords” tab on the top of the “Have I Been Pwned” website and type in any password, especially those used across different sites.

If it’s been “seen,” it’s time to change the password on sites where it is used, and stop using that password altogether.