UNESCO says:
The site comprises 244 islands, islets and coastal areas that are located in the Gulf of California in northeastern Mexico. The Sea of Cortez and its islands have been called a natural laboratory for the investigation of speciation. Moreover, almost all major oceanographic processes occurring in the planet’s oceans are present in the property, giving it extraordinary importance for study. The site is one of striking natural beauty in a dramatic setting formed by rugged islands with high cliffs and sandy beaches, which contrast with the brilliant reflection from the desert and the surrounding turquoise waters. The site is home to 695 vascular plant species, more than in any marine and insular property on the World Heritage List. Equally exceptional is the number of fish species: 891, ninety of them endemic. The site, moreover, contains 39% of the world’s total number of species of marine mammals and a third of the world’s marine cetacean species.
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