The United States Navy maintains a number of its ships as part of a reserve fleet, often called the "Mothball Fleet". While the details of the maintenance activity have changed several times, the basics are constant: keep the ships afloat and sufficiently working as to be reactivated quickly in an emergency. In rare cases, the general public may intercede for ships from the reserve fleet that are about to be scrapped – usually asking for the Navy to donate them for use as museum ships, memorials, or artificial reefs.