Sky Island: Being the Further Adventures of Trot and Cap'n Bill after Their Visit to the Sea Fairies is a children's fantasy novel written by L. Frank Baum, illustrated by John R. Neill, and published in 1912 by the Reilly & Britton Company—the same constellation of forces that produced the Oz books in the first decades of the twentieth century.

Sky islands are isolated mountains surrounded by radically different lowland environments. This has significant implications for natural habitats. Endemism,  altitudinal migration, and relict populations are some of the natural phenomena to be found on sky islands.