Down to a Sunless Sea is a science fantasy novel by American writer Lin Carter, the fourth in his Edgar Rice Burroughs- and Leigh Brackett-inspired series The Mysteries of Mars. It was first published in paperback by DAW Books in June 1984 and reissued in hardcover and trade paperback by Wildside Press in February 2008. The title of the book is taken from a line of the poem Kubla Khan by Samuel Taylor Coleridge. David Graham wrote a post-apocalyptic novel with the same title (ISBN 0-7091-7836-0), also derived from the same Coleridge poem.