What is the difference between CC0 and the Public Domain Mark?

Use CC0 when you, the rights holder, want to actively dedicate your work to the public domain — waiving every right you legally can, worldwide.

Use the Public Domain Mark when you are identifying a work that is already free of known copyright restrictions (for example because copyright has expired or it was created by an entity that didn't hold copyright). The Mark doesn't change the work's status — it just labels it.

If you're a creator releasing your own work, CC0 is the right tool. If you're a librarian or curator pointing at an old work, the Public Domain Mark is.