If you are finished with a project, but you want to keep it around for future reference, you can archive it by clicking on the archive icon for the project, on the right hand side of your My Projects dashboard. To unarchive a project, first click the Archived filter on the left hand side of your My Projects dashboard, and then click its archive icon a second time to unarchive it.
Archived projects are never removed, and they continue to count towards your disk quota.