Sky islands are isolated mountains surrounded by radically different lowland environments. The term originally referred to those found near the southern borders of the US States of Arizona and New Mexico with the northern borders of the Mexican States of Chihuahua and Sonora, and has extended to similarly isolated high-altitude forests. The isolation has significant implications for these natural habitats. The American Southwest region began warming up between ∼20,000–10,000 years BP and atmospheric temperatures increased substantially, resulting in the formation of vast deserts that isolated the sky islands. Endemism, altitudinal migration, and relict populations are some of the natural phenomena to be found on sky islands.