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The USS Monitor was an American ironclad that was in service with the United States Navy in the mid-19th century. In the American Civil War, the Monitor took part in the historic Battle of Hampton Roads, the first battle between ironclad vessels, when it stood off against the Confederate's own ironclad CSS Virginia.
A painting depicting this battle was stored in the USS Enterprise's library computer in 2254, and was among the materials viewed by the Talosians when they scanned the Enterprise computer. (TOS: "The Cage")