The Calusa were not farmers. They were hunter-gatherers who built a kingdom on the wealth of the sea. From Mound Key, which most experts identify as their capital city of Calos, they controlled a territory that stretched from Tampa Bay in the north to the Ten Thousand Islands in the south and east to Lake Okeechobee. The mounds they constructed were not random heaps of waste. They were engineered structures -- platforms for temples, residences for rulers, and burial memorials for the dead. Water courts and canals threaded through the island, and the features of this infrastructure are still visible today. The Calusa created similar shell midden islands up and down the southwest coast, but Mound Key was the ceremonial and political heart of their world, the place from which their king exercised authority over a network of subordinate chiefs and villages.

The Spanish arrived on Mound Key in the 1560s, and the encounter between European ambition and Calusa power played out on this artificial island. King Carlos hosted a political wedding here, giving his sister -- later baptized as Antonia -- in marriage to Pedro Menendez de Aviles, the Spanish adelantado, in a calculated alliance. In 1566, Spain's first Governor of Florida was appointed on the island. A fort and settlement followed, along with a Jesuit mission founded by Father Juan Rogel called San Anton de Carlos -- the first Jesuit mission in the Spanish New World. In 2020, archaeologists confirmed the fort's location on Mound Key and identified it as the oldest known North American example of tabby concrete, a building material made from burned oyster shells. But the alliance between the Calusa and the Spanish was fragile. Frequent conflict led to the Spanish abandoning the island in 1569, just three years after they arrived.

The Spanish left more than ruins behind. They brought diseases -- smallpox, measles, influenza -- to which the Calusa had no immunity. Over the next two centuries, epidemic after epidemic swept through the population. Combined with ongoing warfare and the disruption of their traditional way of life, these diseases effectively ended the Calusa civilization by around 1750. The island they had spent millennia building fell silent. In the years that followed, Mound Key was used by pirates, Cuban and Portuguese fishermen, and American pioneers. Frank Johnson and his wife, known as Grandma Johnson, received a homestead on the island in 1891. She taught subsequent settlers to fish, to farm, and to build houses that could withstand Florida's brutal summers. By the turn of the twentieth century, most settlers had moved upriver to Estero.

The Johnsons sold Mound Key to the Koreshans in 1905. The Koreshans were followers of Cyrus Teed, who had established a utopian community in nearby Estero based on his belief that humanity lived inside a hollow Earth. After Teed's death in 1908, the community declined, and in 1961 the remaining Koreshans deeded Mound Key and their other properties to the state of Florida. The island was added to the National Register of Historic Places on August 12, 1970, and it is now administered by the Koreshan State Historic Site. Approximately nine acres remained privately owned by the McGee family until 2019, when they agreed to sell their remaining parcel to Lee County. Today, the island is accessible only by boat, launched from the Koreshan State Historic Site or Lovers Key State Park.

There are no docks, no restrooms, no concession stands on Mound Key. Visitors arrive by kayak or motorboat across the open waters of Estero Bay and tie up along the shoreline. A trail spans the width of the island, with interpretive displays explaining the Calusa engineering that created the ground underfoot. Walking across Mound Key is walking on thousands of years of human activity, compressed into a landform that rises and falls with the contours of ancient mounds. One hundred and thirteen of the island's one hundred and twenty-five acres are managed by the state park system. The shell beneath your feet was placed there by people who dominated this coast for more than two millennia, who hosted Spanish governors and Jesuit priests, and who built an island from the sea itself before the diseases of another continent brought their civilization to an end.