One of the largest ever collections of breached data, including 773 million email addresses and over 21 million passwords, has been discovered.

The details: The 87-gigabytes breach, discovered by security researcher Troy Hunt, is made up of many different individual breaches from thousands of sources. It was found in a popular hacking forum, which Hunt chose not to name. He’s dubbed the leaked files “Collection #1.”

What you should do: If you want to find out if your email address has been compromised, head to the website Have I been Pwned. If it hasn’t, you can breathe a sigh of relief. If it has, you should probably change your password right away. While you’re at it, you can also check if any of your passwords have been leaked here. Don’t forget to choose a secure password as a replacement.

How’s that for size: Thankfully, this information is confined to email addresses and passwords, and doesn’t include sensitive information like credit card details or health records. However, for sheer size it’s virtually unprecedented. The hack on Marriott affected 500 million accounts. It seems the only data leak larger than this is the breach of 3 billion Yahoo accounts back in 2013.