Beginning roughly 2,000 years ago, the Calusa enjoyed centuries of dominance as the undisputed rulers of southwest Florida. Theirs was a complex society with trade routes spanning hundreds of miles; a powerful military; and built works including wide canals, islands made of shells and towering buildings.

Unlike the Maya, Aztecs and Inca, the Calusa built their kingdom, which stretched from modern Tampa Bay to Ten Thousand Islands and as far east as Lake Okeechobee, without agriculture.

Researchers have long wondered how a society that collected all of its food by fishing, hunting and gathering was able to secure enough food to support its ambitious construction projects and military might. Now, a new study published in the Proceedings of the National Academy of Sciences reveals details of how the Calusa stockpiled live fish in massive holding pens, or “watercourts,” built out of oyster shells. The idea that these watercourts held fish is not new, writes George Dvorsky for Gizmodo, but the paper is the first to conduct a systematic analysis of the ancient structures.

The remains of these watercourts—the largest of which is seven times larger than an NBA basketball court—are located near Fort Myers in Mound Key, where the Calusa’s capital city of Calos stood for 500 years.

Mound Key is quite an accomplishment in and of itself. A human-constructed island made primarily of shells, the island’s building materials, by volume, could fill 200 Olympic swimming pools, Victor Thompson, lead author of the new study and an anthropologist at the University of Georgia, tells Haaretz’s Ruth Schuster. The Great Pyramid of Giza is made of roughly 1,000 swimming pools worth of stone, but as Thompson points out, “The ancient Egyptians didn’t eat the stones before they built it.”

The watercourts flanked a 100-foot-wide canal that bisected the entire island. Each one had a roughly six-foot-long opening onto the canal. The researchers speculate that this feature may have been used to drive fish into the pens before sealing them inside with a gate.

For the new study, researchers analyzed two watercourts to determine when and how they were built, how they worked, and whether their appearance mirrored other significant developments in the Calusa kingdom. The team used core samples, excavated fish bones, radiocarbon dating and remote sensing to probe the watercourts for answers.

Radiocarbon dating placed the construction of the watercourts between 1300 and 1400 A.D. This timeframe coincided with the second phase of construction of Calusa king Caalus’ manor—a massive building that could hold 2,000 people at the time of its completion, according to Spanish documents.

The watercourts could also have been an innovation prompted by a drop in sea level that occurred around 1250, potentially impacting “fish populations enough to help inspire some engineering innovation,” says Karen Walker, study co-author and an archaeologist at the Florida Museum, in a statement.

Bones and scales excavated from the ancient holding pens belonged to mullet, pinfish and herring, all schooling species that might have been easily herded inside.

Remote sensing yielded a 3-D map of the island’s surface that features what appear to be ramps leading from the watercourts to two shell mounds—perhaps facilitating the transport of food.

Excavations found ancient ash and other evidence pointing toward the presence of racks for drying and smoking fish, according to the statement. And core samples from the watercourts contained a layer of dark gray sediment that appears to be on par with ancient pond scum. The researchers say this suggests the water inside of the structures did not circulate much, and that the walls were tall enough not to get flooded by high tide.

“We can’t know exactly how the courts worked,” says Michael Savarese, study co-author and a geologist from Florida Gulf Coast University, in the statement. “But our gut feeling is that storage would have been short-term—on the order of hours to a few days, not for months at a time.”

The Calusa built their entire way of life around the ocean and estuaries of the Gulf Coast, creating a vast empire by learning to manipulate their environment. Though eschewing agriculture once led some researchers to assume that the Calusa were less sophisticated, it also made them innovative and unique.

“The fact that the Calusa obtained much of their food from the estuaries structured almost every aspect of their lives,” says Thompson in the statement. “Even today, people who live along coasts are a little different, and their lives continue to be influenced by the water—be it in the food they eat or the storms that roll in on summer afternoons in Southwest Florida.”