Caldera de Taburiente National Park, Canary Islands, Spain
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Some peaks of this isolated archipelago in the East Atlantic off of southern Morocco have the advantage, along with the Atacama and Mauna Kea, of being above a so-called temperature inversion layer. That means that a "sea of clouds" forms below the peaks, keeping moisture low and the atmosphere free of star-distorting turbulence. Perhaps the best island is La Palma, whose Caldera de Taburiente National Park hosts a 7,800-foot-high rocky mound called Roque de los Muchachos that's home to numerous observatories.