Do you put the Rime of the ancient Mariner in Quotations or underline it because its a relatively long poem?

Even though it's a long poem, use quotation marks. (You'd use underlining or italics only for a book-length poem. "The Rime of the Ancient Mariner" was originally published as one of the poems in a collection of works by the author, Samuel Taylor Coleridge, and his friend William Wordsworth.)