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In 1517, indigenous fishermen watched warily as Spanish ships dropped anchor off the mangrove-lined shores of southern Florida. Only a quarter century had passed since Christopher Columbus and his crewmen first landed on an island in the Bahamas, but word of the foreigners’ hunger for land, slaves, and gold had spread along the coasts of the Caribbean and the Gulf of Mexico. In Cuba, 385 kilometers south of Florida, Spanish forces had recently taken brutal control of the island, enslaving many of the indigenous Taíno. So when 20 Spanish soldiers and sailors waded ashore in southern Florida to replenish their ships’ water supplies, the local inhabitants were ready.

One of the Spanish soldiers, Bernal Díaz del Castillo, later chronicled the battle. Florida’s indigenous fighters, Díaz noted, “had immense sized bows with sharp arrows, lances, and spears—among these some were shaped like swords—while their large powerful bodies were covered with the skins of wild beasts.” The attack was swift and ferocious. The first volley of arrows alone injured six Spanish soldiers. The rest barely escaped with their lives, fleeing back to the ship with the water they so badly needed.

The fighters who drove off the Spanish so effortlessly in 1517 were almost certainly Calusa, a powerful indigenous group who built an early kingdom along the Florida coast from just south of Tampa Bay to the Keys, and who exacted tribute from other native groups in the region. Unlike most ancient complex societies, however, the Calusa realm depended on the riches of the sea, not agriculture. The Calusa fished for mullet, pinfish, pigfish, and many other species along the Gulf Coast estuaries, and harvested rich shellfish beds. They dug extensive canal systems for their canoes and heaped shells high to create mounds that served as foundations for enormous houses and other important buildings. Whether Díaz realized it or not, the battle he fought was one of the first clashes between two nations—the Spanish and the Calusa—and it would be the Calusa who would prevail in this conflict for more than 200 years.

Now, scientists are trying to better understand who the Calusa were and how western Florida’s coastal environment fostered their powerful kingdom. To answer these questions, archaeologists are exploring a site that served as the Calusa’s capital city and was also one of their greatest architectural achievements. The site, called Mound Key, is a 51-hectare artificial island that the Calusa constructed almost entirely from oyster, clam, and other shells possibly as early as 1,700 years ago. It sits in the shallow waters of Estero Bay about 38 kilometers south of Fort Myers. For the past two years, Victor Thompson, an archaeologist from the University of Georgia, and colleague William Marquardt at the Florida Museum of Natural History, have been working at Mound Key with a team of researchers to uncover the island’s secrets.

Thompson stands behind the steering wheel of a pontoon boat, threading it between the mangrove islands of Estero Bay where pelicans, herons, and wealthy retirees all compete for the estuary’s abundant fish. A strong, steady wind threatens to knock Thompson’s cowboy hat off his head, but never succeeds. Thompson likes to keep it light, making banter and jokes during the 15-minute trip to Mound Key. As he edges the boat into a narrow channel surrounded by mangrove trees, the wind dies and a mullet jumps in the still water. Thompson ties the boat to a tree next to a sign informing visitors that they have come to a State Historic Site.

Thompson thinks that the Calusa designed the landscape of Mound Key with care, almost as if they had a blueprint in mind. The two tallest mounds, he explains, lie roughly at the center. In between, Calusa workers carved out a large canal that divided the island into two, and each half was sprinkled with several smaller mounds. Spacious, multi-family houses seem to have stood on the summits, providing homes for an estimated 2,000 people when Mound Key’s population was at its peak. “I imagine you would have these gleaming white mounds everywhere,” Thompson says, as he leads the way along a trail, and “the buildings had to be huge.” In all probability, the king’s residence perched on the tallest shell hill, dubbed Mound 1 by archaeologists. Towering almost 10 meters above the water, the big mound would likely have been the tallest human construction in sight.

On either side of the canal, archaeologists have discovered large square pools known today as “water courts.” Mud and mangroves now fill these pools, but Thompson hypothesizes that the Calusa once stored live fish in them—a ready supply of very fresh seafood.

To investigate how the Calusa built the island, Thompson and his colleagues took more than 20 core samples from across the island last year and analyzed them. The archaeologist concedes that the picture is still far from clear, but he thinks the island began as a small shallow spot in Estero Bay, possibly guarded by mangroves. Over time, he says, ancient coastal dwellers may have stopped by, shucking oysters and clams there and tossing away the shells until they created a small mound. Later, the Calusa seem to have regarded this white mound above the waves as an important place. They chose to settle there, recycling shells as building materials, and constructing and sculpting Mound Key’s impressive landforms. Thompson estimates that it took between 350,000 and 500,000 cubic meters of shell and other material to build Mound Key, or roughly one-fifth the estimated volume of Egypt’s Great Pyramid. “The key difference is they didn’t eat the Great Pyramid before they built it,” he says. Even today it is an impressive statement of the power wielded by the Calusa kings.

Thompson was drawn to the site because it promises to answer big questions about how such a large kingdom developed without the benefit of agriculture. Complex societies need to produce enough food to support a large population, and organize labor for things like building irrigation systems or, in the case of the Calusa, constructing Mound Key. As a rule, says Thompson, such societies rely heavily on agriculture to create the surpluses needed to feed their people. The Calusa, however, harvested few plants other than chili peppers and squash. Yet they built a kingdom powerful enough to hold off Spanish colonists for more than a century and created enormous architecture by relying on food gathered from Gulf Coast estuaries. “I really wanted to understand what was the history behind that,” Thompson says. Mound Key, the Calusa capital, was a natural place to start.

A path leads from the boat landing to the top of the large mounds. On the trail, Thompson spots a small clamshell with a punched hole. He stops to pick it up, explaining that it was a weight for a Calusa fishing net. An early archaeologist in the region, Frank Hamilton Cushing, found pieces of ancient fishing nets at Key Marco, an island about 55 kilometers south of Mound Key that was inhabited by the Muspa, a group once ruled by the Calusa. In all likelihood, Thompson says, the Calusa used very similar palmetto-fiber nets, with gourds and pieces of wood strung along the upper edge as floats, and perforated shells and stones along the lower edge as weights. As they waded through the estuary, holding the top of the net at the water’s surface, they dragged the weighted bottom along the estuary floor, leaving few escape routes for the fish. “This represents the base of the Calusa economy,” Thompson says, holding the punctured clamshell. “This is part of the technology that allowed them to create surpluses and feed the people.”

But what first set the Calusa on the road to developing a powerful kingdom? At the Florida Museum of Natural History in Gainesville, archaeologists William Marquardt and Karen Walker have been intensively studying that question for 30 years. Their research has focused on another large Calusa site, known as Pineland, which lies about 30 kilometers northwest of Mound Key on Pine Island Sound. Marquardt thinks that Pineland may have preceded Mound Key as the Calusa capital. Its layout resembles that of Mound Key in some key respects. Each has two large complexes of shell mounds separated by a central canal.

The people of Pineland, like those of Mound Key, were dependent on a shallow water estuary—Pine Island Sound—to provide a steady supply of fresh seafood. “It’s a broad, flat, grassy estuary that is surrounded and protected by barrier islands,” says Marquardt. “In most places it’s no more than two feet [0.6 meters] deep.” Seagrasses, mangroves, and mudflats combine to provide protection and rich habitat for fish and shellfish, making the estuary a prime fishing ground.

The Calusa took up residence at Pineland around 100 CE and they remained there into the 1700s, but Walker and Marquardt’s work shows that they sometimes moved the settlement as sea level fluctuations altered the shoreline. After 850 CE, however, construction at Pineland boomed, as the Calusa constructed high mounds and dug large canals, including one that cut between the two largest mound complexes and flowed for four kilometers. These waterways likely functioned much like streets in a modern town, only for canoes instead of cars. “Remember, these are water-oriented people,” Marquardt says, “they aren’t driving carts and horses.”

Based on their research, Marquardt and Walker think that the Calusa began consolidating and extending their political control over southern Florida during the 1200s, as the climate became more favorable. During this period, another neighboring coastal group, the Tocobaga, were also rising to prominence, and the Calusa lords may have organized their people to counter this threat. Even so, Marquardt thinks that the real Calusa state was born nearly 300 years later, when the Calusa rulers became “conscious of the Spaniards in the early 1500s.”

The Calusa had little trouble fending off the early Spanish sailors and soldiers, but the foreigners proved persistent. In 1566, a Spanish admiral named Pedro Menéndez de Avilés sailed to Florida’s southwestern coast. Menéndez had already founded the city of St. Augustine on the Florida coast, and his plan in 1566 was to establish a new colony and a Jesuit mission. He arrived at Mound Key with a show of force: 200 Spanish soldiers. But he left most of the soldiers outside the ruler’s house, taking just 20 men inside to meet the Calusa king.

The admiral and his men found about 1,000 of Mound Key’s inhabitants there. According to one account, the Calusa sang and performed dances. The king himself, a man named Caalus, kissed Menéndez’s hands, a seeming sign of submission. Caalus then asked to take Menéndez as his “older brother,” and offered his sister’s hand in marriage. Menéndez accepted both suggestions in the interests of diplomacy, and persuaded Caalus to let him build a fort that doubled as a Jesuit mission on Mound Key.

Thompson’s research at Mound Key has now turned up new evidence of the relationship between Caalus and Menéndez. By constructing a map of the island with publicly available lidar data (an imaging technique that uses airborne lasers to make detailed contour maps of ground surfaces, even ones that are covered by dense vegetation), Thompson first zeroed in on the most important residential areas. He then probed the tops of the two largest mounds and other parts of the island with ground-penetrating radar. At the summit of Mound 1, the radar revealed a line of post holes that appear to be remains of Caalus’s residence, the place where the Calusa sang and danced when Menéndez arrived.

On the island’s second largest mound—Mound 2—Thompson located another line of post holes, and last year, he and a team of archaeologists excavated a two-by-two-meter-long trench there. The digging was slow going because the ground itself was mostly made up of shells and since the Calusa used shells for making all sorts of tools—from axes and hammers to net weights—everything the team dug was a potentially significant artifact. “You want to know why no one has dug here?” Thompson asks. “It’s because you have to be out of your mind.” But the painstaking work paid off. The team discovered pieces of Spanish pottery, as well as a mold that Spanish soldiers used for casting lead shot needed for an early firearm known as an arquebus—artifacts likely left behind by Menéndez’s soldiers and priests.

Despite the early signs of friendship, Caalus came to resent the presence of the mission in his capital. The Calusa already had a well-developed religion. Mission records showed that the Calusa believed that each person had three souls—one that was their shadow, a second that was their reflection, and a third in the pupils of their eyes. So the Christian idea of one person, one soul, was never widely accepted. Moreover, Jesuit efforts to convert people were a direct challenge to Caalus’s authority. The Calusa believed that their rulers possessed a strong direct connection to the Calusa gods. So Caalus soon began scheming to kill the Spanish, but his plots came to nothing.

Menéndez’s plan to convert the Calusa and set up a profitable colony was a dismal failure. He withdrew his people from that part of the Florida coast in 1569. More than a century later, a Franciscan order tried to establish another Catholic mission at Mound Key, but the Calusa were no more interested in converting then than they had been during Menéndez’s time. And when the missionaries began unloading hoes for tilling the earth, the Calusa, proud coastal dwellers, scorned the idea of working in fields. Agriculture was for slaves.

But the Calusa’s defiance came at a steep price in the end. Rival indigenous groups acquired firearms from British colonists in the 1700s, giving them a lethal advantage over the Calusa, who were armed only with bows, arrows, and maces studded with shark teeth. When British slavers in the region offered other native groups, such as the Creek and Yamasee people, a musket for every captive they brought in, they frequently turned up with Calusa men and women. “Indians from Georgia are coming all the way down to Key West and beating the hell out of the people who are left,” says Jerald Milanich, an archaeologist at the Florida Museum of Natural History, describing the raids. The destruction of the Calusa “happened in two generations, maybe even one. It was a horrific thing.”

Still the survivors refused to bow to the Europeans. To escape captivity or death, they finally fled south to Cuba from their estuary homes.

The last known record of the Calusa was dated 1763.

Thompson brings the boat around to the north end of Mound Key and finds a place where the mangroves haven’t grown too far out from the island’s shore. He sits on the edge of the deck and slides into the knee-deep water. “I’ve never been here before,” he says. The lidar map is not as detailed in this part of the island, but it does show some long, narrow strips of land, or rather shell, that the Calusa had built out into the water. Thompson clambers through the thicket of mangrove branches to the island’s unexplored edge. He walks along the strip of land, dodging spider webs and ducking under more tree branches. He thinks the area may contain graves, or more water-logged perishable artifacts like the net fragments found at Key Marco. Next year, Thompson plans to excavate the water courts near the main canal and possibly some of the unexplored parts of the island.

He pushes through the trees and shrubs onto low ground where mangrove shoots push up through the soil like stubby gray fingers. The walk is slow going, and the island stretches out behind the curtain of plant life. There is a lot more of Mound Key to be explored. For Thompson, the search to understand the Calusa’s unique and complex society lies out there beneath the mangroves and seashells.

Correction: A previous version of this article misidentified the shell hammer in one of the above photos as a pick and erroneously included 500 settlers accompanying a Spanish admiral named Pedro Menéndez de Avilés when he sailed to Florida’s southwestern coast in 1566.