Projects can be archived, instead of being deleted. Unlike deletion, projects can easily be retrieved from archive.
To archive a project: select "Archive project" from the Project page drop-down menu.
To unarchive a project: choose the Team Settings link from the drop-down menu in the navigation bar, and select the Archived projects tab. Choose the project you'd like to restore, and select the "Unarchive project" from the Project page drop-down menu.