Mound Key in Estero Bay is a shell-built island created over more than 2,000 years by the Calusa, whose engineered mounds, canals, and water courts expressed political, ceremonial, and burial traditions. Most researchers identify it as Calos, capital of the Calusa kingdom in southwest Florida. It was also a major early contact site between the Calusa and the Spanish: King Carlos hosted a dynastic marriage there, and in 1566 the Spanish established a fort, settlement, and Jesuit mission before abandoning the site in 1569 after conflict. Later used by settlers and others, the island became a state park and archaeological site of exceptional significance.