Encrypting filesystems

I’d like to know if there’s a way to encrypt a file system after it has been formatted and filled with data.
I’ve got several unencrypted disks in my computer atm and I don’t like that much at all. The reason is I got these disks years ago when encrypting stuff was hard and I didn’t know how to do it. I can’t take backups of the data on the disks and reformat them either, there’s just to much data(around 400 GiB).