Experts think they have figured out how the ancient Polynesian inhabitants of Easter Island, or Rapa Nui, put those enormous red stone hats on top of the island’s famous giant statues.

Anthropologists photographed the cylindrical hats, known as pukao, and used the photos to make 3-D models they could analyze in depth. They concluded in a new study that the hats were likely rolled atop the statues using large ramps, in a technique called parbuckling.

Located nearly 2,000 miles off the coast of Chile in the Pacific Ocean, Easter Island is famous for the nearly 1,000 enormous stone statues, known as moai. The massive works were erected there starting around the 13th century A.D. by the Rapa Nui, an ancient Polynesian society that later collapsed mysteriously. Scientists have long puzzled over how, with limited people and resources, the island’s inhabitants somehow transported the huge, heavy statues from rock quarries across the entire island.

Not only that, but many of the statues wear enormous cylindrical hats known as pukao, measuring up to 2 meters (6.5 feet) in diameter and weighing some 12 metric tons each. Carved from quarries of red scoria, or volcanic rock, the pukao were also transported across long distances, then somehow placed atop the maoi, which are up to 10 meters tall.

While earlier research focused on how the Rapa Nui “walked” the statues by rocking them slowly back and forth, the new study zeroed in on exactly how they got the pukao on top of the giant heads. Led by researchers from Binghamton University, State University of New York, the study concluded that the pukao were probably rolled from the quarry to where the moai were located. After that, their work with the 3-D models suggests the hats could have been rolled up large ramps using the parbuckling technique.

“In parbuckling, a line would have been wrapped around the pukao cylinder, and then people would have pulled the rope from the top of the platform,” Carl Lipo, professor of anthropology at Binghamton, explained in a statement. “This approach minimizes the effort needed to roll the statue up the ramp.”

Lipo and his colleagues, including experts from Penn State, the University of Oregon and the University of Arizona, published their findings this week in the Journal of Archaeological Science.

The new study is only the latest chapter in scientists’ fascination with Easter Island and the Rapa Nui civilization. The island’s population is thought to have numbered as many as 20,000 at one point, but had dwindled to fewer than 3,000 by 1722, when the first Europeans arrived. One theory suggests the construction of the statues led people to cut down many of the island’s trees, causing soil erosion that made the island inhospitable to wildlife and farming. A 2016 study, in which Lipo was also involved, analyzed weapon-like artifacts found on the island and concluded they were likely common tools, disputing the idea that warfare destroyed the island’s civilization.

Tragically, the Europeans brought smallpox with them that further decimated Easter Island’s population, and by 1877, only 111 inhabitants remained. The Rapa Nui had largely disappeared, leaving behind only the enduring mystery of their giant stone statues and their massive hats.